Gas Gun Barrel Break-In

In a former life I spent 28 years as a full time Pennsylvania Game Warden. In tracking down poachers and solving various game law infractions I often asked the same questions as any other law enforcement officer: Who, What, When, Where, and How? Let’s use that approach with regard to the topic of gas gun barrel break-ins. Who? We gunsmiths can either do the break-in ourselves or leave it for the customer. Most of us in the profession are busy, real busy!

While we probably don’t want to add such tedious tasks to our already full plates, we are the professionals and have many tools that shooters don’t have or need. One such tool that is helpful to properly breaking in a gas gun barrel is the bore scope. As professionals we have a greater understanding of how easily damaged the bores of our firearms are, especially if they are made of stainless metal. This extra knowledge and care, coupled with tools to observe and avoid unintended damage, makes our firearms better.

There is another, somewhat selfish reason for the gunsmith to break-in the barrel. Regarding the Bay of Pigs, JFK noted, "…victory has 100 fathers and defeat is an orphan." Would any of us want to see a real tack driver leave our shop only to come back a few days later ruined by an improper break-in? I have seen it happen! Who is that gun owner going to blame? One of two "orphans" being either the barrel maker or the gunsmith who installed it.

He will not blame himself and, in his defense, damaging a barrel via improper cleaning is easy to do yet hard to detect that he probably won’t even know he was at fault. So, at the very least, if we don’t tackle the job we owe it to our customers to inform them of the proper tools and techniques to do the job right. What? What are the goals of a proper barrel break-in? To burnish the pores of the metal forming a smooth surface less prone to snag at the bullet.

Smoothing out any angular tool marks left as a result of the chambering process. Removing any burrs at the gas port caused by the drilling process. "Above all else, do no harm" to the barrel while achieving the first three goals. That misquote is often attributed as part of the Hippocratic Oath. While it was said by Hippocrates, circa 460 BCE, it was not apart of the oath itself. Nearly twenty five centuries later, I think that in our jobs as gunsmiths it is still a very good work ethic to embrace. When?

The very term "break-in" alludes to the answer to this question. A breakin needs to be done starting with the first shot through the barrel. Even a few shots before beginning the process will exacerbate the frustrations, time, effort and danger of doing harm to the barrel. If you area gunsmith who doesn’t elect to do the whole break-in process but likes to function fire the gun (a good idea) please bear this in mind.

My advice in that event is to proceed as follows, assuming you have already double checked your assembly, weighed your trigger and used the appropriate headspace gauge. Don’t assume that the barrel came to you from the maker free of metal chips, cutting oils, dirt or other contaminates that could damage it. With a quality bore guide in place flush the barrel with a clean patch saturated with Windex without ammonia. Then run about four dry patches.

Charge a magazine with two rounds, keep the muzzle pointed in a safe direction at all times and begin with the safety on. Lock the magazine in place with the bolt forward, pull the bolt completely rearward and release it smartly (don’t ride it forward). If the bolt was locked to the rear, simply chamber the first round by tripping the bolt catch. With the muzzle still pointed down range, attempt to fire around. The gun should not discharge.

Rotate the safety to the fire/semi position and fire the first round only into a safe backstop while holding the trigger rearward. Keeping the muzzle at the backstop, slowly release all pressure on the trigger. With issue triggers and many match versions there will be a metallic click signifying the disconnector has done its job. Observe that the second round has successfully chambered.

While keeping the muzzle pointed in a safe direction, put the weapon back on safe, eject the chambered round (the magazine should lock the bolt to the rear) and continue to clear the rifle by removing the now empty magazine. I grant this isn’t the most thorough function firing but you have checked many important things. The magazine has successfully fed from both sides and held the bolt to the rear when it has been run empty. You verified several trigger, safety and disconnector functions.

The rifle has extracted and ejected a fired case which you can use to visually verify signs of pressure or any overt problems. I usually give this case to the customer to show that I successfully fired the gun without blowing my head off.

Congratulations, at this point, you are poised to begin the actual break-in. The first shot of the break-in is almost always the most difficult in terms of getting the copper out. If you elect not to proceed with the break-in you need to let your customer know that he needs to thoroughly get the copper from your test fire removed from the bore before he fires the next shot. We need to talk briefly about salt bath nitriding as it has become popular with AR barrels.

The smart folks that apply this procedure advise that you thoroughly break-in the barrel before sending to be processed. From personal experience I can tell you that all traces of fouling need to be removed after firing the last shot. If you fail to get all the copper out the nitride will go on right over it. When the barrel is sent back and you bore scope it, any trace copper that you missed on your last cleaning will really show up, far more so than before the coating process.

Scrubbing that trapped copper out will be difficult and you run a greater risk of bore damage. Also, once you have removed the copper deposit you may have a small section of barrel that failed to be sufficiently coated with the nitriding chemical. Such a section could tend to accumulate copper in the future. Before sending any barrel out to be coated be especially meticulous in your final cleaning and bore scoping to ensure you are sending the cleanest possible barrel for processing.

When you get the barrel back don’t rush out to the range and immediately fire the gun. It will perform horribly! To get the barrel to shoot right run several patches wet with Windex without ammonia through the bore. Follow that with a quality clean bore brush also wet with Windex. It won’t hurt at all if that bore brush has been previously used as long as you have thoroughly cleaned it.

You will find that whatever bore brush you use the first time will have a lot of resistance, even more so if it is new, and there will be much cleaning rod flex that can damage the bore quickly. A used brush will cut down on the flex a bit. After brushing, use more patches wet with Windex. It is not uncommon for the black nitride compound to bleed out during cleaning for months after. Don’t be concerned about that. After about three wet patches go to dry patching.

Then head for the range and expect groups to be pretty much normal right from the get go. Where? From Star Trek episode "The Devil In The Dark," one of Dr. Leonard "Bones" Mc Coy’s famed quotes was, "I’m a doctor, not a brick layer!" My degree is in Wildlife Management, not Chemistry. I did have to take a bunch of boring chemistry classes, however. I don’t remember much from them except Bunsen burners and that most chemical reactions take place faster in higher temperatures.

Unless you have your own private range adjacent to your work shop as I do, you will probably be firing at a public outdoor range and you might not be allowed to occupy a firing position for hours at a time while you wait on solvents to attack the copper. Chemical solvent reactions will take place faster in summer temperatures than they will at freezing 32 degree winter temperatures.

Many copper solvents "tell" us when they are accomplishing their purpose by discoloration of the bore patches but the solvents can commonly give false readings in the winter, and sometimes even in the summer, by showing patches with no bluish or greenish color. You can be fooled into thinking all the copper is out when, in reality, you just haven’t been patient enough for the slow moving chemical reactions to take place.

I recommend following the particular solvent manufacturer’s recommendations on how long to soak the bore. Just remember in colder temperatures to go to the absolute time limit because the chemicals will be reacting slowly. It is important at any time of the year to verify what the patches are telling you by actual bore scope observations.

When you think you have all the copper out, verify it in the bore scope before firing that next shot or you’ll be fooled and need to go back to cleaning to get persistent copper deposits completely removed. I bore scope for all ten shots religiously. Getting careless or rushing to the next shot before removing every last trace of copper from the previous shot is defeating the purpose of breaking in a barrel.

We professional gun builders can’t tell our customers that they have to wait until summer for delivery when it is easier for us to break in a barrel. What you may be able to do, however, for winter break-ins is get the gun into a heated environment between shots, such as a range shack or club house, permitting the solvents to do their job quicker and more effectively. You can also rely more on abrasive cleaners in the winter to speed break-in.

I don’t like that because the back and forth scrubbing motion of abrasives (not the actual abrasives themselves) can cause rod flex and damage to the delicate bores. The first thing I do when a shipment of barrels arrives is break them when a customer sends me his upper to install anew barrel I can just pull one out of storage and get his finished product back quickly.

If you live in the north where winters are long and you don’t have good range facilities, batched summer barrel break-ins may be an option you might want to consider. How? Armorers have to know quite a bit about a wide variety of guns, ammo and shooting topics. Barrel makers only need know everything about their subject. Most big barrel making companies have web sites discussing barrel break-in and other topics.

I have studied many of these sites and one commonality seems all of them talk only about bolt action rifles and ignore gas ports. Many of them focus on the back of the barrel in the throat area where those angular ridges are likely

present. If you follow most of the barrel maker’s web site directions you will likely get the back 2/3 of your gas gun barrel properly broken in while leaving the front seven inches or so loaded with residual copper and not properly broken in at all! The majority of barrel makers recommend a ten shot shoot and clean method. Fire a shot, thoroughly clean out the copper and repeat for a total of ten times. A few advocate continuing after that with additional shots with more rounds fired between cleaning.

Two shots, five shots, etc. If you follow this approach you will find that the first shots will be difficult to clean and some time around the seventh to tenth shot copper cleans out easier. That is a good sign but doesn’t always happen. Even in those instances where the barrel starts responding earlier I continue to shot ten and don’t take short cuts. Be sure your cleaning procedure is not causing any damage to the delicate rifling.

Some people have the extra patience to go a couple of additional shots if the barrel fails to respond by shot ten. More power to them! Barrel maker’s recommendations for cleaning between shots are roughly divided in half between two fairly distinct camps. The first camp advocates solvent only cleaning, maintaining that any use of abrasives during break-ins is counterproductive and will open up the pores in the metal that you are trying to burnish.

The second camp often recommends abrasives in connection with solvents. Their theory is that the mere passage of ten bullets alone won’t be enough to work down those ridges that the chambering caused and that the abrasives help "sand down" the roughness. The emphasis of the second camp is mostly in the back of the barrel while the first camp addresses all of the barrel. Which camp is right? I agree with parts of both and tend to blend them together depending on the specifics of the barrel I am working with.

From the first camp, I like the idea of burnishing the pores of the whole barrel and limiting cleaning as much as possible to solvents. From the second, I don’t fear the use of abrasives but have seen damage caused by flex in cleaning rods when short stroking abrasives. Using abrasives for break-ins or general cleaning at the back of the barrel is probably less risky. Two-thirds of the way down the barrel in the area of the gas port entails more rod flex danger to the barrel.

I do agree with the second camp that ten shots won’t likely smooth out all the chambering ridges at the back of the barrel. Instead of using abrasives there, however, I prefer to fire more rounds in what I call the "extended break-in." Equipment With the exception of a bore scope, you won’t need a lot of fancy or expensive tools to break-in a gun barrel. I rely on a bore scope for barrel break-ins and assume gun building professionals already have one, though the cost may be prohibitive for hobbyists.

Even an entry level bore scope used with the absolutely essential 90 degree mirror attachment will cost more than the most expensive barrel. Adding just a few options can drive the cost of a bore scope to double that of a quality rifle barrel. While the rest of the necessary equipment is more common and less expensive, it must be spot on. A quality one piece straight rod of the correct diameter for the caliber is a must. Don’t use a .22 caliber rod to clean a .30 caliber firearm.

The smaller the caliber of rod, the more flex with more potential to damage. One of my two Anschutz squirrel rifles is a .17 HMR. The cleaning rod I have for it is so skinny and flexes so much that I can’t even drive a bore patch down the barrel from the breech. I have to very carefully insert the naked rod through the muzzle, then put a patch on the end of it at the breech and pull it back through the bore. It takes me forever to clean that thing.

The rod needs to be just long enough to accommodate the length of barrel you are breaking in. The rod I use for 20" Service Rifle AR barrels is shorter than the one I use for 26" barrels on Match Rifle ARs. If I were to use the longer Match Rifle rod to clean the shorter 20" Service Rifle barrel I would surely run a much greater risk of rod flex and accompanying damage to the barrel.

Also make sure you have a clean shop rag or paper towel to wipe the rod down frequently keeping it clean. Next, you need a quality bore guide. In Stoner platform guns these replace the bolt carrier assembly and some are tight fitting with O-rings to help center them up. The best are made out of metal.

I recommend either JP Enterprises (JPRifles.com) or Wheeler Engineering (WheelerEngineering.com) models and avoid the cheaper plastic ones due to an incident that happened out at Camp Perry Commercial Row at my store there during the CMP Service Rifle National Matches a couple of years ago. A competitor came to me with an AR barrel that he said shot out prematurely. I bore scoped it and the back was almost completely void of normal fire cracking, telling me the modest round count was true.

I noticed, however, that the lands were uneven at the back with regard to where they started. I dropped a throat gauge in the back of the barrel and it went in clear to the handle! As I continued to scope it I noticed other evidence of cleaning rod damage. I asked him to fetch his cleaning stuff. He returned the next day and I inspected everything. He had been using a plastic rod guide designed for the AR. No moving parts, what could go wrong with that? His coated rod was the right length and not too long.

I verified that it was straight and that the handle rotated freely. He said he used J-B Bore Cleaning Compound and wiped off the rod with a rag frequently. Finally, in desperation, I picked up the rod guide a second time and looked a little closer. The hole in the guide for the .22 caliber rod didn’t look right. I dug out a set of digital calipers and measured it at almost 0.35!

The very rod guide that should have prevented barrel damage was undoubtedly accountable for letting his rod go crooked into the back of the barrel, damaging it. I gave him my last new JP bore guide made out of metal in exchange for his worn out plastic unit. Come see it in person out at Perry in the summer during CMP Service Rifle week at Building 910-A. You can bet I will also have quality metal bore guides for sale.

He said he had used the worn out plastic one for over twenty years and that it had "lasted" for many barrels. Nothing felt abnormal, such as scraping the sides of the back of the barrel, during cleaning. Shooters can damage barrels without even realizing they are doing so. We armorers can too! For rifles like the old M14/M1A you can buy bore guides that fit over the front flash suppressors.

Most are made of plastic so I recommend occasionally measuring the inside diameters of these to make sure they aren’t getting too worn to keep the rod going straight. If you think the back of the barrel is easily damaged by a cleaning rod going in crooked, you don’t even want to think about what will happen to the crown of the muzzle if subjected to similar rough handling. You’ll also need a fair quantity of the right size bore patches, solvent of your choice and Windex without ammonia.

Why the Windex and why does it need to be the harder-to-find variety sans ammonia? I am indebted to my friends at Northern Competition for this tip. The Windex is used near the end of each cleaning to kill two substances that may be present in the bore. Many bore solvents are petroleum based and their residue can contribute to carbon fouling. Carbon fouling is best removed with abrasives and we are trying as much as possible to not use abrasives during the break-in process.

The solvents that you will be using will have lots of ammonia in them to remove the copper. I am told that trace ammonia left in the bore can combine with chemicals in the burning of gunpowder to form a third substance that can be corrosive to the barrel steel. What chemical solvent should you use? I use Hoppe’s #9 Bench Rest Copper Bore Cleaning Solvent to remove copper during normal cleaning after break-ins. It says on the bottle it can be left in overnight.

While it isn’t as fast acting as some other copper solvents, it is more forgiving. Because I normally clean indoors due to my shop being on my private shooting range I am always involved in other gunsmithing chores while I wait on solvents to react with the copper. If a fifteen minute gunsmithing project turns into thirty minutes I don’t have to worry about Bench Rest damaging a fine rifle bore. I am sure Bench Rest would be just fine for breaking in bolt guns.

For gas gun break-ins, however, I have been using the most potent copper solvents on the market, stronger than Hoppe’s Bench Rest, because of what occurs at the port. Enhancing AR-15 Break-In We’ll start discussing the differences between .22 centerfire five and six groove barrels compared to three and four grooves. Each barrel maker is free to set his own groove-to-land ratio and there are differences between makers. 2:1 is common, meaning the groove is twice as wide as the land.

The ratio would be roughly 66%:33%. RKS Enterprises in Canada, makers of gain twist blanks, uses this ratio. 70%:30% is also common, meaning the groove is just a little more than twice as wide as the land. Bartlein uses this ratio in both their 5R and 4 groove barrels. Brux uses a wider land in their regular barrels. Their ratio is 60%:40%. The width of their lands is cut in half on their narrow land option, resulting in a 80%:20% ratio. Most barrel makers don’t state their ratios on their web sites.

Some will tell you if you inquire while others hold that information closer to the vest, though I don’t see any logic in not giving this information out to professionals. The goal is to get the gas port located in a groove and not cut the lands. All other things being equal, that goal is easier if the groove is wider, such as in a 70%:30% ratio or even bigger 80%:20%. The number of grooves is also key.

For the most part, when you try to "crowd" six lands and grooves into a .22 caliber bore the actual groove widths are going to be more narrow

than most five groove barrels. The US military uses six groove barrels in the M16, guaranteeing the gas port will cut a land. A full discussion on what’s wrong with G. I. barrels and how to make them shoot "more better" is a full article in itself. Generally, you can expect grooves to be wider still in four groove barrels and even wider in three groove barrels.

I inquired with a popular maker of five groove .22 centerfire barrels whose groove-to-land ratio is 70%:30% and was told that the width of their grooves is on the order of 0.098". Most AR-15 gas ports measure between 0.093" and 0.099", making it almost impossible to drill the port without at least cutting the edge of a land. Try drilling such a port in the more "crowded" six groove barrel and you are nearly guaranteed to cut a land. Compare this to a four groove barrel where the groove-to-land ratio is 70%:30%.

In that case your groove is going to be much wider at 0.123", a much easier target to try to center that gas port inside. A three groove would likely be even easier. Remember NCIS Special Agent Leroy Jethro Gibbs Rule #8: "Never assume!" Opting for the wider 0.123" groove of a four groove barrel doesn’t mean you can just poke a 0.093" hole in the top of it and automatically guarantee that you won’t hit a land! Its not that easy.

Properly locating the port in such a barrel requires that a "feeler" trace a groove from its origin at the muzzle back to the position you are going to drill. This is probably not a job for hobbyists. If your barrel maker doesn’t know how to do this and doesn’t guarantee that your port will be located in a groove I suggest you select another barrel maker. Krieger Barrels guarantees that their AR-15 gas ports will be drilled in a groove on four groove barrels that they do in their shop.

There’s good news for you folks shooting bigger caliber gas guns like the M1A. Generally speaking, .30 caliber barrels are going to have wider grooves. I stated above that a 5R barrel in .223 Remington may have a rather narrow groove width of only about 0.098". A .308 barrel (5R) from the same maker has a much wider groove at 0.138". On top of that, the M1A gas port is smaller at only 0.078", making centering the gas port in such a groove is much easier. What’s The Big Deal? G. I.

M16 barrels have six grooves, guaranteeing that a land will be cut when the gas port is drilled. Obviously, the military doesn’t think that cutting a land is a big deal. Well, look at their accuracy standards, such as they are. I am assuming that shooters and armorers reading this have higher accuracy expectations than the military. When a land is cut it forms a surface similar to a potato peeler. As the bullet spins past that surface, tiny specs of copper are shaved off the sides of the bullet.

Those tiny pieces of copper are propelled forward by the same gas that is pushing the bullet. After only one shot I frequently see a solid layer of copper laid down in the top of the bore from the gas port forward for a couple of inches. The width of that deposit will be about the same as the diameter of the gas port. It is very difficult to get out. The problem will normally be worse if the leading edge of the land is cut in drilling the port versus the trailing edge.

In right hand twist barrels the leading edge of the land as viewed from the rear is to the left. Since gas ports in most AR barrels are about in the forward third of the barrel it is easier for me to locate them in the bore scope if I come in from the muzzle, in which event the leading edge of the land will appear to your right. If you ignore this problem and don’t remove this heavy copper deposit it may act to pull more copper from other bullets passing over it making the buildup increase.

Obviously, shaving copper jacket material off your bullets will have ill effects on accuracy. There are two ways to deal with removing this copper. One carries a high probability of damaging the barrel. My first inclination when I found these heavy copper deposits in front of the gas port was to use abrasive J-B Bore Cleaning Compound on a patch and "short stroke" it in about three inch passes just forward of the gas port.

I found that I had to do a lot of scrubbing to get all the copper out so I could fire the next break-in shot. Given the amount of back and forth scrubbing needed I became concerned that cleaning rod flex could be damaging my lands. Even though I use a high quality metal bore guide I could see in the bore scope that the cleaning rod was flexing into my lands just forward of the chamber and wearing them prematurely and unevenly.

In terms of success this method was smoothing out the surfaces of the bore near the port to the point where at the end of a ten shot break-in the barrels weren’t accumulating that streak of copper. However, the damage that the back and forth scrubbing action was doing to the rear of the barrel was enough to negatively impact accuracy so I knew I had to modify my technique. Plan Bconsisted of switching to the most aggressive chemical copper solvent on the market.

Sweet’s 7.62 Bore Cleaning Solvent did a good job but Montana X-Treme Copper Killer 50 B. M. G. was even better! First, flush out the gas port and chamber with some kind of aerosol such as Birchwood Casey Gun Scrubber. Do not install the front sight housing or gas tube. Using a quality bore guide, follow with a patch soaked with Windex without ammonia. Dry out with about four clean dry bore patches. Fire the first shot into a safe backstop.

Replace the bolt carrier assembly with the bore guide and start patching out the bore with solvent. While doing so, position the upper receiver upside down so the solvent will pool around and forward of the gas port where the

most copper will be found. Continue to patch until the bluish discoloration of the bore patches subsides. Let the upper sit with the gas port still pointed down for about 15 minutes and try another patch wet with solvent. It will probably show some coloration as an indication that copper still remains. Let it sit another 15 minutes and repeat. At some point your patches will start coming out without any color and that should indicate that all copper has been removed.

Shoot a shot of Gun Scrubber into the gas port to flush excess solvent out and run a patch of Windex through the bore. Dry patch and then inspect the bore with your bore scope, paying particular attention to the throat area and the gas port. If all traces of copper has been removed, it is time to fire the second shot. Don’t be surprised, however, if you find copper, especially near the gas port. If the slightest bit of copper remains don’t shoot the next shot. Go back to the solvent.

If a lot of copper remains you may want to saturate the bore with the 50 B. M. G. solvent and follow with some Montana X- Treme Copper Cream. As a general rule, don’t mix cleaning solvents as it may create unintended compounds harmful to the bore, however, mixing one Montana X- Treme products is fine. You can also follow regular Hoppe’s No. 9 with their Bench Rest, but don’t follow a Hoppe’s product with Shooter’s Choice or a Montana X-Treme solvent, or vice versa.

The Montana X-Treme Copper Cream contains no abrasives and is meant to be worked back and forth like J-B. Do this judiciously, avoiding as much rod flex as possible. Only resort to the cream in cases where the copper is particularly persistent and you are unable to completely remove it with solvent. When you think you have all the copper out this second time, flush everything as before and verify with the bore scope. Fire the next shot and repeat the cleaning procedures described above for each.

After the tenth shot clean everything up and assume most of the break-in has been successfully accomplished. After shot ten, start cleaning the bore normally. The copper won’t tend to hold as tenaciously as during the break-in, so drop the excessively "hot" solvents and go back more moderate ones like Hoppe’s products. The pores of the metal will be somewhat burnished by the break-in and you should use a bore brush every time you clean to get the hard carbon fouling out.

Extended Break-In A lot of barrel manufacturers will tell you that a mere ten shots isn’t nearly enough to fully break-in a rifle barrel. I agree and believe the figure is closer to 200 rounds, though this will vary a lot between barrels. There are a few things I recommend during this extended period. First, clean a little more frequently than normal. If you normally shoot 80 to 100 shots in a practice session, consider taking short break-ins mid session to clean the bore during the first through the barrel.

Try to not over heat the barrel by firing rapid fire strings during this extended break-in period. With fast twist barrels (1:8 or 1:6.5) I recommend never firing more than two back-to-back ten round rapid fire strings. Any more is a waste of ammo because the fast twist barrels heat up so much, giving off a lot of barrel mirage, tricking your eye and resulting in misplaced groups, false zeroes and fat groups. Limit your rapid fires to one string during the extended break-in period.

The "registration point" is sort of a memory that the barrel develops. After the harmonic whip subsides, a gun with a good registration point will bring the muzzle back to the same point where it was before the shot was fired. Barrels with faulty registration points will not return to the same point, resulting in fat groups and wandering zeroes. Some barrel manufacturers believe that overheating barrels early in life can result in less repeatability of their registration points. Why run the risk?

Keep the barrel cool for the first couple hundred rounds and be on the safe side. Making It Easier Work smarter, not harder. We have all heard that phrase and it applies to breaking in gas gun barrels. Barrett Tillman and Derrick Martin wrote the very informative The Complete Guide to AR-15 Accuracy. In it the difficulty of locating gas ports in the grooves of AR-15s due to narrowness is discussed. At the time, the best advice was to just shoot off burrs.

This can happen during the official ten round break-in but may require more shooting in severe cases. Getting those gas port burrs worn clear down along with any angular ridges caused by the chamber reamer at the back of the barrel are what we are accomplishing during my suggested 200 round extended break-in. to avoid having to shoot off burrs is to not create them in the first place.

Many barrel makers make the initial gas port drilling slightly smaller than specification and then ream it out to the final diameter in the belief that doing so will help avoid burrs. Martin mentioned in the book that he makes his ports at 0.099". The military specifies theirs to be 0.093" and Krieger Barrels and other custom shops use that size. I prefer the slightly more narrow 0.093 myself and find that most rifles will function well at that size.

If I identify a gun that doesn’t I can always ream the port to the fatter size. The skinnier 0.093 port may miss cutting a land that the fatter port would nick in a few cases. Another approach is a device called an E-Z Burr (ezburr.com). Their web site has a short video demonstrating how the tool works. The device consists of a retractable spring loaded carbide cutting blade.

Drill the gas port to your desired diameter and push the E-Z Burr in from the top of the gas port. When the cutter contacts the top of the hole the blade will be pushed back into the arbor (the thin shank.) When the arbor is pushed sufficiently far enough down into the bore to clear, the cutter will spring back out into the cutting position. As you retract the spinning cutter it will automatically remove any burrs.

As it enters the port further it will collapse back into the arbor until the tool reaches the top and comes back out. It takes seconds for the entire process. After you have flushed out any metal chips by spraying a high pressure aerosol like Gun Scrubber down from the top of the gas port, run patches wet with non-ammonia Windex through the bore and dry it out with about four clean and dry patches.

When you observe your handiwork in the bore scope later you should see a nice chamfer all around the circumference of the inside surface of the port. Other times you may see less of an obvious chamfer. What you will hopefully not see are any remaining burrs. A handy test for burrs at the gas port, regardless is using the E-Z Burr or not, is to push your rod with a clean dry patch slowly past the port. In the case of large burrs you may be able to feel the patch hesitate as the burr grabs at it.

Usually this will result in some white patch fabric clinging to the burr just at the gas port and this should be readily visible in the bore scope. There are two very small issues with the E-Z Burr. First, in the case of where the gas port nicks or cuts through a land, some minor chatter may occur in the cutting of the E-Z Burr and you can actually raise a burr that wasn’t there to begin with. This is very rare but easily explained.

The video on the E-Z Burr site shows the tool being used on a flat surface like a thick piece of metal. We are working on a curved surface inside the bore. On top of that, the land sits above the groove by about the same thickness as a sheet of paper. If the gas port cuts a land the E-Z Burr is trying to do its business on a curved surface with two irregular heights. Chatter can occasionally happen. I elect to use the E-Z Burr because this is rare and most barrels are improved by the deburring process.

In the few exceptions where I have seen chatter raise a small burr it always shot off within the first couple of break-in shots. Regarding the actual chamfer itself, its sloped surface is below the groove and the groove is normally cut at the same diameter as the bullet (0.2240") so any chatter on the sides of the chamfer shouldn’t ever contact the bullet surface anyway.

If you opt for three or four groove barrels and religiously locate the gas port in a groove you will get a really nice job with the E-Z Burr, creating a fairly even chamfer around the circumference of the hole and seldom raising a burr. This, in turn, will cause less copper to accumulate forward of the gas port, making your break-in much faster and simpler and risking less cleaning rod damage to other parts of the barrel. Regarding the second issue, look at the photo of the E-Z Burr.

You will notice that the distance from the top of the cutter blade to the bottom of the arbor is 0.24". You must have that much empty space inside the bore of the barrel in order for the blade of the tool to pop back out into the cutting position before it is rotated back up through the gas port on its return journey. The problem is that you don’t have that much space inside a .22 centerfire bore!

If you use the tool as it comes from the factory it will jam down through the top and not stop until it bottoms out on the reverse side, hitting the bottom of the bore, damaging it, and the blade will still not come out. To fix, simply grind off enough of the bottom of the arbor to achieve a safe clearance. I recommend beveling the finished product to mirror the original contour and remove any sharp edges.

In the event that you don’t feel confident in making these modifications contact E-Z Burr directly to have this as custom service performed at their factory. Remember to set up your drill press to limit the downward travel stopping the tool before it bottoms out on the far side of the bore. The stock number of the tool you will want to order is the Micro Deburring Tool #CAR0880-B. If you plan to work on AR-15 carbines they have smaller gas ports and will require a tool with a smaller diameter arbor.

The M14/M1A, whose gas port lies between the standard AR-15 and the carbine, would require a third tool of a different size. Conclusions Properly breaking in bolt gun barrels is pretty simple and straight forward. The risk of damaging them is considerably less. Because of the gas ports, breaking in AR barrels is more time consuming and requires greater attention to detail. If you have to use a back and forth scrubbing action up around the port you will have cleaning rod flex which can damage the barrel.

Use the right equipment and the tips in this article to make the job easier and less prone to damage. The time you spend doing a very slow non-invasive break-in when the barrel is brand new will more than make up for itself over the life of the barrel in easier cleaning. Properly broken in barrels also shoot better. The key, however, is to start with atop quality barrel to begin with. Don’t expect the best break-in to compensate for a poor quality barrel. AG