AR-15 Chamber Selection, Part Three

It’s no surprise that every rifle has its own specific sweet spot where it wants ammo to be seated. There is no such thing as a universal seating length or bullet jump. This must be worked out for every rifle built or rebarreled as it is important to group size. My customers receive a free box of ammo with his gun seated to the sweet spot of his barrel. All he has to do is measure the rounds and set his die to duplicate.

In the decade that I was the Armorer for the Army Reserve Marksmanship Program’s Service Rifle Team I gave each shooter a hand press with a competition seating die fixed with Loctite. It was the same press I used in the test shed the day I established his sweet spot. We loaded all of our 80 and 90 grain ammo over length so shooters could use their hand press for the rifle in question and final seat. Idiot proof! Six hundred yard scores for the Team skyrocketed using this approach.

Since you don’t have to "chase the throat" (a technique used with VLD bullets to constantly lengthen the seating depth to compensate for erosion) with boat tail bullets, the sweet spot I establish initially works for the life of the gun. I think a tune up is good at the end of the shooting season to establish anew, longer, seating length but it is optional. If you get out to Camp Perry during the CMP Service Rifle phase of the Nationals, stop in my store on Commercial Row (Building 910-A) and visit.

I have pictures of test targets showing the value of working out this sweet spot seating length. I’ve seen an average group size reduction of 33% from random seating to sweet spot seating and enjoyed a 50% reduction with the guns I maintained for the Army Reserve. I have worked out sweet spot lengths for hundreds of chambers ranging from the short original SAAMI spec to the long NATO spec.

When I compared groups with long range 90 grain Bergers in short and long chambers I couldn’t see any improvement in the long chambers. Those comparisons were all done at the end of the bullet seating sweet spot work ups. Getting bullet seating sweet spot right is more important to tight grouping than the length of the chamber. Soft Seating Soft seating is used by some bolt gunners where ammo is seated long like the Reserve Team has done, with the cartridge shoved into the chamber and the bolt closed very gently.

This jamming effect causes the bullet to be pushed down further into the casing while engaging the lands at the back of the barrel. Remember I said that by providing a custom press and die for each firearm I fielded for the Reserves I pretty much "idiot proofed" the seating process? No disparagement meant, but leave it to Joes to find away to muck things up. Some of my guys got lazy and decided not to use the tools provided to take a short cut.

They instead hard seated, shoving the long rounds into the chamber and letting the bolt slam forward. If you don’t believe this is hard seating, stick your finger in there and release the bolt! The bullets seated and there were no slam fires but performance was awful! Chamber pressures went way up—a real safety issue considering we were already at a max load, causing some primers to be blown. If a cease fire was called to unload, opening the bolt pulled the bullet and spilled powder everywhere.

Groups were fat and the incidence and severity of fliers went through the roof. Hard seating is a bad idea and something you want to admonish your customers against regardless of which chamber they are using. Ordering Custom Reamers After a couple of years of ordering Wylde chambers from barrel makers and getting the hodge-podge I described in Part Two, I decided it was time to get my own custom reamer.

I had worked with Dave Kiff from Pacific Tool & Gauge (pacifictoolandgauge.com, before on custom orders with good luck. There are other good reamer manufacturers out there but my experience is with Dave and Pacific so I’ll describe the information I provide him with. First, establish the specific chambering, such as .223 Remington or 5.56mm NATO, and what kind of guns are you going to be chambering. Gas guns that are going to be magazine fed can’t have chamber dimensions too tight or reliability will suffer.

Second, talk about the neck. Your reamer maker will want to know what brand of brass you intend to use (some brands are thicker than others) and if you intend to outside neck turn the brass. It is common to tighten the neck diameter in chambers where the shooter is just firing one handfed round at a time. Examples may be long range shooters that never use the magazine. Those necks can be about 0.003" tighter. Third, if your chamber has a name like Compass Lake, Wylde, Holliger or NATO, provide that.

Not all reamer makers will be familiar with the specifics of all these chambers so you will still need to go into further detail. Then establish the leade or throat angle. Fourth, there is a pilot bushing out on the very front of the reamer and its size is very important. Your reamer company will need to know if the lands in your barrel are industry standard at 0.219" or tall like Krieger normally supplies at 0.218". Bartlein offers both land options.

Brux uses a tall 0.218" in their narrow land barrel but a standard 0.219" in their regular land width barrels. Most other barrel makers use a 0.219" land. Most barrel companies make their grooves 0.2240" unless you order something different. If chambering barrels with differing land heights, get pilots in both sizes as they interchange easily. Fifth, the most common leade diameter is 0.2240" but you should listen to suggestions if your reamer maker thinks there is a need to vary.

Leade lengths vary and they conspire with the throat angle to determine overall length of the cartridge. The best way I know to get this right is to mail the reamer manufacturer a dummy cartridge loaded to the exact length you want and let them cipher it out. Dave Kiff actually suggested this to me, I have done it every time and he gets it spot on with zero error! If I were talking to some other reamer maker and they weren’t receptive to this idea, I would hang up real quick!

Trouble Shooting If you get a gun in to work on and the customer doesn’t know what chamber is in it you can get some rough ideas by knowing what goes on inside the chamber during firing. The case, which is part of a loaded round that enters the chamber undersized, is caused to expand to the size of the chamber upon firing due to the pressures generated by the burning powder, a temporary situation called obturation. The brass has a memory of sorts.

As the bullet moves forward and the gas pressure subsides the brass contracts about 0.001" toward where it originally started. This is called spring back. By simply asking the customer for a few pieces of his fired brass you can measure the diameters of the case body and neck, add 0.001" to allow for spring back, and get a good idea of what these dimensions are in his chamber. You already know you can take a Stoney Point reading to determine overall length.

The gun doesn’t care what the original length was when it was new, only what the chamber length is now. A barrel starting with a Compass Lake chamber and firing thousands of rounds, eroding forward to measurements similar to anew NATO chamber now performs, pressure wise, like a NATO chamber. Longer chambers generally pressure up less regardless of whether they are born long or whether the length has come about by throat erosion.

I have remarked many times that one of my least practiced skills was diagnosing malfunctions. That’s a good thing! Stoner’s platform is a remarkably well engineered weapon. Most malfunctions that do occur are related to bad magazines, dirt or lack of lube, and bad ammo. When customers come in complaining of feeding or extraction problems, right after ruling out dirt or improper lube it is always good to consider their ammo if they are reloading.

If the shooter is hand loading I like to fire several rounds of factory new ammo in his gun and see if there are problems. If my ammo works it’s a good indication the customer may not have his reloading dies set up properly. The diagnostic tool for this is a Headspace Gauge Kit from Hornady. It is caliber specific and fits on the comparator which is seated on the bottom leg of a digital caliper. Pick up a few rounds of brass that successfully fired in the customer’s chamber and measure with the Headspace Gauge.

Remember spring back and add 0.001" to get the true dimension of the chamber in question. Take measurements of the customer’s ammo the same way. The hand loads need to come out 0.003" shorter than the chamber or they may not feed or extract properly. This "shoulder bump" produces ammo 0.003" shorter in the shoulder than the chamber.

Full length sizing and neck sizing are accomplished by different dies but both dies can bump the shoulder back by the required 0.003", usually accomplished by screwing the body of the die down closer to the top of the shell holder. Follow the manufacturer’s di-

rections for this. There can be mismatches between dies and shell holders. Over decades of loading I have found situations where the bottom of the die came down into hard contact with the top of the shell holder but didn’t give adequate shoulder bump. If you ever run into such a situation work down the top flat surface of the shell holder, permitting the die to go down further. I have had good luck, function wise, just neck sizing my slow fire rounds in the AR-15.

However, if you gauge the customer’s ammo has the shoulders set back the correct 0.003" you might try full length sizing or checking the trim length to make sure it isn’t too long. In some of the older ARs chambered with SAAMI reamers I have seen a number that needed brass trimmed 0.010-0.015" shorter than the loading manuals call for. Polish Every Chamber Last, but not least, polish every chamber. I am indebted to Glen Zediker’s great book, The Competitive AR-15 Builders Guide, detailing how to perform this.

You will need a Brownells Rifle Polishing System with .357 hone (#080-608-338) and Flex-Hone oil. You can follow the instructions in Glen’s book or the directions that come with the hone. Two points that seem to be missing from both, however, are cleaning before and after. The chamber needs to be spotless to begin with because you don’t want to pick up any debris that can scratch the chamber.

After polishing you need to spray out the abrasives from the chamber and barrel extension with some high pressure aerosol, use a chamber brush and spray again. After everything has dried, run a couple of patches soaked with Windex without ammonia through the bore and then dry patch. When breaking in gas gun barrels, after firing I soak the bore with a very strong copper-removing solvent and turn it upside down, allowing solvent to pool around the gas port, which is a problem area that likes to hold copper fouling.

Because I don’t want to get excess solvent running down inside my front sight housings I leave them off during the break-in shots. That means I am extracting all fired casings by hand using the charging handle. I find that I have to apply quite a bit of muscle to extract brass from unpolished chambers. The same barrel with a polished chamber lets casings come out very easily, about half the effort. If that doesn’t tell you something about the effectiveness of this process nothing will.

It takes longer to dig out the items you need than it does to perform the polish. It is that easy! Do it with every gun you build. If you take in a gun built by another armorer and the customer is complaining of function issues I suggest looking at the chamber with your bore scope. Military armorers replace more barrels ruined by improper cleaning or neglect than those genuinely shot out. Rusty chambers are not uncommon and the polishing process can often salvage them.

Preserving The Chamber Rust leads to pitting and this can start within a handful of days after shooting. Corrosion can happen regardless of barrel steel type. One of the best ways to avoid it is to clean the bore, chamber and barrel extension after each firing. Then, if the gun is going to be stored for more than just a few days, apply something to the barrel to ward off corrosion. Brownells conducted a great study on this topic a few years ago (bit.ly/1hh5tt U) and two rust inhibitors got great marks.

WD-40 scored well and is readily available. Boeshield T-9 also earned high marks and is available from Brownells. When cleaning any firearm that isn’t going to be shot again soon, I saturate a clean patch with Boeshield and work it back and forth a few times to burnish it into the pores. Then I spray a good shot of the aerosol directly into the chamber and barrel extension, turn the rifle around and hit the front of the muzzle, crown and flash suppressor.

I have observed the least bit of pitting in the crown will usually spell the death of accuracy. The night before going to the range, spray the chamber and front of the barrel with a shot of high pressure gun scrubber, then follow by running a patch or two of Windex WITHOUT ammonia through the bore and dry patch. I doubt that armorers and gunsmiths will ever entirely agree on the choices of chambers.

Most likely, those advocating long ones will continue to hold onto their reamers until they are pried from their cold, dead hands. Some of those folks make good guns and skillful shooters continue to log winning scores with them. At the end of the day, it won’t matter which chamber a person has in his gun if he points the muzzle at the 9 ring. Only a lot of training will prevent those 9s from being pointed out there in the first place! AG