Building The 90 Grain AR-15, Part One

Two breakthroughs allowed the M16/AR-15 to replace the M14/ M1A as the Service Rifle match winner in recent years. They were the float tube, which prevents applying pressures to the barrel, and high quality, accurate match bullets that are more wind resistant than 55 grain GI ball. The float tube is still with us but when is the last time you saw anyone win with 55 or even 69 grain bullets out to 600 yards?

Just as heavier bullets with higher ballistic coefficients replaced the 69 grainers, we are seeing the new 90 grain projectiles making their presence known in Service Rifle competition. It’s possible a customer is going to approach you regarding converting to use heavier bullets. This series will help you provide guidance to them. Very Low Drag .22 caliber, 90 grain bullets have been around for about a decade.

JLK was the first to introduce them around 2004 and their bullet is still available through Swampworks (swampworks.com), having the highest BC of any 90 on the market. Berger also offers a VLD 90. Many High Power shooters avoid the high maintenance VLD bullets in any weight and don’t want the extra work of "chasing the throat." Sierra introduced their 90 grain boat tail in 2005 proclaiming that it was a major advancement for long range shooting with the AR-15. "Long range" to most of us means 600 to 1,000 yards.

As you are probably aware, boat tails don’t normally require chasing the throat and are relatively maintenance free. The AR-15 is most commonly equipped with a 20" barrel although some "funny gun" match rifles have 26" tubes. Just as people load their 80s to different velocities, folks experimenting with 90s also push the speed envelope to varying degrees, so it can be difficult to compare the two.

Using ballistic programs, I’ve come up with an estimate of about 17% greater "wind proofing" by switching from 80s to to the VLD design shows an improvement of 28.6%. No doubt about it, 90s area better mouse trap! Saying that 90s will be about 17% more "wind proof" than 80s is a bit abstract, so I dug out a ballistic calculation program and arrived at the following example.

Shooting an 80 in a 10 mile per hour full value cross wind at 600 yards results in about 34.5" of wind drift, requiring a 23 click (with ¼ minute of angle sights) correction. A 90 in the same conditions drifts only 29.5", requiring three less clicks. That in itself is no big deal and anyone can reach up and move the sights three more clicks.

If the wind never changed, life would be good, however, if there is any such thing as a range where the wind blows consistently during the twenty minutes of record fire at 600 yards I have seldom experienced such. Where the 90s pay for themselves is when the wind is changing rapidly and unpredictably. Let’s say you have a perfect rifle capable of shooting an Xevery time at 600 and that you area skilled enough to point it there.

Now let’s say that your last shot was a pinwheel X and, as you admire it in the spotting scope, you see no change in conditions. What happens if, as you come off the scope and roll over to execute the next shot, the wind picks up to from 10 to 12 mph and you don’t catch it? If you are shooting an 80 that shot is probably going to come up in the 9 ring. A 90 would have held the 10 ring. Does this happen in real life?

I have an inbox of emails from satisfied customers relating cases where everyone else on the line got caught in a condition change except them. I’m not advocating just lying on the mound, relying on the ballistic superiority of the 90 to "shoot through" condition changes, but it is simply a reality that we all eventually get caught by wind pickups that we don’t see. The 90s will harvest you some points in such situations.

Comparing guns with equal length barrels, 90s shot out of a .223 Remington will be the ballistic equal of anything that can safely be shot out of a .308 Winchester and the 90 will do so at greatly reduced recoil. Visit me on Commercial Row at Camp Perry (I am normally in Building #910-A) to see test targets yourself. 90 grain bullets can be loaded using the existing tooling and shot out of the Stoner platform Service Rifle competitors are already comfortable with.

As a military match armorer for over a decade I had the opportunity to test hundreds of rifles and barrels,

This increases dwell time, helping with primer retention when running hot loads of any bullet weight. and I kept meticulous records.

In analyzing those records I found that 90s, on average, shot groups 7.7% tighter than 80s and did so with no degradation to 77 grain magazinelength ammo performance. 90 grain Berger boat tails, when shot out of quality Krieger or Bartlein cut rifled barrels with 0.218" lands and 0.2245" grooves, were about half as likely to shoot a flier as 80s, and if they did the severity was just about half as much. So why aren’t more people using 90 grain bullets? First, there is a lack of loading data.

If you follow what little load data there is you will not be able to reach speeds that will keep bullets supersonic to 1,000 yards. Current published data is very conservative and many shooters confident in working up loads for their particular rifle when provided an idea on where to start are reluctant to get involved without published recommendations.

I will provide data on specific loads in the later article on reloading that have worked in a variety of barrels with one critical caveat: Start well below any recommendations and work up slowly, watching for signs of pressure. 90 grain bullets require more finesse than loading more user friendly 77s and 80s. Control of things like bullet seating depth, concentricity and neck tension are very important, along with use of a good chronograph. I strongly recommend reading Glen Zediker’s Handloading For Competition.

Another problem, to shoot 90s across the course you need a barrel with a different twist rate. JLK and Sierra recommend a fast 1:6.5" twist barrel to stabilize their long 90 grain bullets. Berger says that 1:7" will work with their bullet, however, I have tried 90s in a few twenty-inch 1:7" bores without much success. They stabilized and didn’t keyhole but just weren’t as accurate as I wanted. Berger’s 90s shoot great out of 1:6.5, however, so I just quit trying the 1:7s.

Buying 1:6.5" barrels permits use of all 90 grain bullet offerings, making the chances of finding a successful load greater. The 90 grain Berger B. T. requires a taller 0.218" land (industry standard is 0.219") and a slightly deeper 0.2245" groove (industry standard is 0.2240".) Only three barrel makers offer these specific options: Krieger, Bartlein, and RKS Enterprises in Canada.

A third issue is the lack of interest with some of the big military teams and attitudes about sharing information vary quite a bit among the more successful teams. Regardless of their official position, information eventually leaks out and trickles down to other shooters. Some teams are fairly helpful while others are closed mouth, especially with regard to load data. Remember, they typically use brand new brass with crimped primers so their loads are often too heavy for civilians.

Some of the military teams also use unconventional chambers that allow loads with higher powder charges as well. The Army Marksmanship Unit shot 90 grain VLDs in the early to mid 2000s, before the boat tail designs were available, using them mostly for 1000 yard matches and not across the course. One of their shooters using JLK 90s broke a 1000 yard Interservice record that I had held for nearly a decade with an M14 and heavy 180 grain bullets.

VLD bullets require "chasing the throat", on going measurement of erosion and increasing bullet seating depth to compensate. The AMU uses their support staff to take these measurements and load everchanging ammo. That had to be a real pain, as there were only about three barrels available at the time, but they did quite well considering the limited options.

In 2005, when Sierra introduced their 90 grain boat tail, it was obvious the AMU wanted to transition to a less maintenance heavy option as they had a pallet of the new bullets. When they watched the Army Reserve Service Rifle Team blowing up these new bullets in early 2006, I think that was the straw that broke the camel’s back and they threw in the towel shortly after that. A few years later the AMU came out with their AR-10s (M110), which they had to petition the NRA to accept as legal service rifles.

Most of the other

308 loads. All were fired in 20" barrels. big military teams tried Sierra 90s at one point or another and had problems with mid air blow ups. Most of the big military teams wanted to use existing technology. While the Army Reserve shared that same desire, they stayed the course and worked with barrel manufactures and bullet companies to market better 90 grain options, leading to the success of 90s today.

A fourth issue, 90 grain bullets have a reputation for bullet failures, with projectiles blowing up mid air somewhere between exiting the muzzle and hitting the target. One manufacturer’s product (packaged in green boxes) has really been the subject of a lot of bad mouthing in the pits and on some websites. This is a valid concern and regardless of how many points 90s may gain by virtue of their "wind proofing", it is nearly impossible to recover from a loss of ten points due to a bullet failure.

I think that in early 2006 when these bullet blow ups were being reported those stories caused a lot of confusion among shooters and all 90s got a bad rap when only one brand was actually guilty. I have never blown up a JLK 90. A few years ago Berger thickened the jackets on many of their VLD bullets and the 90 was one of them.

Current bullets are absolutely reliable, however, 90s became suspect and not much was happening between 2006 and 2008 when the Army Reserve talked Berger into offering a user friendly, reliable, thick jacketed 90 grain boat tail. There are some lesser challenges to shooting 90s as well.

First, since we are spinning these bullets at RPMs faster than with 80s and are often reaching high pressures, especially with 1000 yard loads, bore fouling (both copper and carbon) will usually be evident earlier and likely be more pronounced. This will require earlier, more frequent and vigorous cleaning, or accuracy will suffer. In 1:8" twist barrels, 80 grain bullets commonly run be- American Gunsmithing Association

Turn the rifle on its left side so oil distributes around the rings. Have a rag handy to remove any excess from the carrier that can splatter your shooting glasses. tween 247,500 and 252,000 RPMs while 90 grain bullets fired out of 1:6.5" barrels hit between 288,000 and 293,538 RPMs.

Second, because of the above factors coupled with increased heat, 6.5 twist barrels may wear out faster than normal. "Wear out" in 6.5 twist barrels won’t be just in terms of the length of throat erosion but also in the roughness of it. When I first started working with 90s, some of the barrels, especially those made of an allegedly softer steel used by button rifling manufacturers, were coming into my shop exhibiting early signs of fire cracking at the throat as far as six inches down the bore.

As I bore scoped these barrels I also saw many of them with deeper and rougher erosion around the gas ports, which can also adversely affect accuracy, especially if you are using a thinner jacket bullet. I’ll jump ahead a little and tell you that roughening of the throat, rear of the barrel, and gas port are major contributing factors to bullet blow ups. More specifics on this later. What kind of barrel life can you expect? I hate to put figures on this because it is so variable and depends on many factors.

A couple of years ago I bought a bunch of 1:6" twist stainless button rifled blanks from a respected barrel maker. They were all made from the same run, turned and chambered by the same shop. I fitted two of them to uppers, worked out loads and sent them out to two different shooters. When they left my shop there wasn’t any difference in the throat lengths. The first shooter fired Varget powder in his rifle and cleaned with Hoppe’s.

He has over 1,500 rounds on it (plus the 200 or 300 I put on it developing loads) and sent it in recently so I could reset the bullet seating length to compensate for erosion. He had less throat erosion than the second guy’s barrel and this upper was still shooting the best of any upper I have ever tested of any manufacture, twist rate, shooting any ammo! The second shooter used imported high energy powder in his and cleaned with one of the new "gee whiz" bore solvents.

The barrel was literally destroyed in 500 rounds (plus what I put on in load development, which I estimate to be about 200.) The throat erosion was deeper than the sister gun and it shot like crap! When I was shooting the high energy imported powders in my button-rifled, chrome-moly barrels I have had any number of them go out at about 500 round counts.

Now that I have switched to more conventional powders in these barrels I haven’t had a single one go out at anything less than 1,000 and some are pushing nearly double that. Barrel makers that use one of the cut rifling processes frequently say that their steel is harder and that their products will last longer. I was always skeptical of some of these claims until I got into this project. I have to say, my cut rifled barrels are lasting noticeably longer.

I hope to get 2,000 rounds out of them and maybe double that out of some. Gain twist seems to be helping with longevity a lot. By starting to rotate bullets slower at the back of the barrel they move away from the throat quicker and the hot erosive gasses don’t spend as much time attacking and eating the throat. A third factor is barrel heating. Spinning bullets faster causes more friction and thus more heat. Powder charges tend to be heavy and that adds to the heat problem.

I would be aware that barrel mirage may be more severe with 6.5 twist barrels, regardless of bullet weight. Fast twist barrels would certainly not be my choice for shooting the Infantry Trophy Team Match! Some barrels, perhaps more so with stainless unless it has been stress relieved, tend to "walk" (shift zero) when they heat up and this can certainly be a problem with a 6.5 twist that heats up faster. In addition to potential zero shifts, accuracy can degrade as some barrels heat up.

I recommend when practicing with a 6.5 twist to not shoot more than two back-to-back strings of rapid fire before sitting the gun aside for at least one minute of cool down per round fired. When shooting long

range slow fire and need to extract around because of a cease fire or change in conditions, I recommend placing that round last in the firing queue so it has time to cool. It takes very little time in these hot chambers to change the burning characteristics of the powder. This is a good habit to form even for those shooting 80s, especially later in the string when barrel temperatures are running higher regardless of twist rate. High temperatures will dry out any lubricant on the bolt carrier assembly much faster.

I recommend starting each match or training session with an immaculately clean bolt carrier assembly, properly lubed. Try supplementing that with a drop or two of quality gun lubricant before shooting your 300 yard sighters and again before 600 yard slow fire. Consistent bolt carrier velocity is key to positioning the cartridges uniformly in the chamber and applying the same pressures to the case head each time.

Accuracy in any Stoner rifle can improve by keeping the bolt carrier properly lubed regardless of bullet weight and twist rate. A fourth issue is primer retention. If you are going to keep your VLD 90s supersonic to 1,000 yards it is recommended to start them at 2600 feet per second. Because the Sierra boat tail bullet has a lower B. C. you need to start it at 2650. Achieving these muzzle velocities out of 20" service rifle barrels will require pretty heavy powder charges which result in high chamber pressures.

Brass life is going to be very short, sometimes only one firing. Even military shooters with access to crimped primed cases will occasionally find a spent primer lodged either in the barrel extension or under the trigger mechanism. Either one will disable the rifle and take a fair amount of time to remove. Use of bolt carrier weights, such as David Tubb’s Superior Shooting Systems design (davidtubb.com), may help to lengthen dwell time, keeping your bolt locked up a few milliseconds longer.

This can result in at least getting the casing clear of the ejection port before the primer falls out. As with anything else in the gun business, follow the manufacturer’s recommendation, but I favor using carrier weights only during prone slow fire stages and removing them for rapid fire. Any modification of the bolt carrier assembly can affect both zeroes and barrel harmonics. If you are going to use weights make sure they are present both during zeroing and load work ups.

The most effective way to control chamber pressure is by use of either again twist barrel from RKS (six groove) or Bartlein (5R or 4 groove.) With either, specify a tall 0.218" land and deep 0.2245" groove. Starting twist in front of a Wylde chamber should be 1:13" finishing at the muzzle at 1:6.5". Another effective design is the four groove Brux Narrow Land. Since using these three barrels I have been consistently able to reach 2,600 FPS of muzzle velocity in 20" barrels with no primer issues.

No carrier weight is required with any of these barrels. Such a starting velocity will bring a 90 grain Berger through the 1,000 target with a strong remaining 1,249 FPS (the speed of sound is about 1,100.) Accuracy is stellar in all three. The Match Rifle shooter has other options for controlling dwell time. In those rifles, gas ports can be drilled two inches further forward, increasing dwell time (how long the bolt stays locked) up and primer retention is of very little concern.

I start my gain twist 26" Match Rifle barrels at 1:14" just in front of a Wylde chamber and finish at 1:6.8" at the muzzle. No carrier weight is required and 90 grain Bergers exit at 2,700 and arrive at 1,000 yards at a very strong 1,349 FPS. Next time we will talk about barrels for use with 90 grain bullets. AG