May 19, 2019
Photos by Mark Fingar
Two of the most misunderstood components of the AR-15 are the bolt and bolt carrier. If you surveyed 1,000 AR-15 owners, I bet over 950 of them would choose a Mil-Spec bolt and bolt carrier over any of the truly enhanced products available today. Keep in mind, “enhanced” is more than just putting a high-speed, low-drag coating on a bolt carrier or bolt. That does not qualify as an enhancement. That’s a cosmetic touch.
There are two manufacturers, however, that have made significant contributions to improving the AR-15’s bolt carrier group (BCG) as designed by Jim Sullivan in the late 1950s. Many attribute the AR-15 to Eugene Stoner, but that is not correct. Stoner designed the AR-10 (he was a .30-caliber man), and he left the creation of the AR-15 to Jim Sullivan.
SureFire teamed up with Sullivan to design their Optimized Bolt Carrier Group. Karl Lewis of Lewis Machine and Tool (LMT) also developed an enhanced bolt and bolt carrier. Both of these products have very unique features that improve the reliability and durability of the AR-15.
History of the BCG
According to Sullivan, the original AR-15 was designed to work with ammunition having a chamber pressure of 50,000 pounds per square inch (psi), a 20-inch barrel and a rifle-length gas system. Very few of these original parameters see much application today. Ammunition is produced at higher pressures, and barrels and gas systems are shorter.
Sullivan said that the Colt engineer behind the development of the XM177 carbine had a good solution for the shorter barrel and gas system: a heavy buffer. The XM177 had an 11½-inch barrel with a moderator attached to the muzzle. The moderator was necessary because the muzzle blast of the short barrel was considered excessive for military use. The combination measured a total of 14 inches. However, it didn’t take Colt long to realize the moderator was more trouble than it was worth (it clogged easily with bullet and powder particulates).
When the military resurrected the carbine program in 1985, they told Colt they wanted a 14½-inch barrel for use on the new carbine. This barrel length could accommodate the M203 grenade launcher and bayonet when paired with the carbine-length gas system. That was also enough barrel to solve the muzzle blast problem, but Colt had to do some testing before settling on the .0625-inch gas port diameter.
The problem with the government’s list of specifications is the short gas system operated at significantly higher pressure than the system Sullivan designed. Colt may have done a great job of making what the military wanted, but the fact remained that the rifle Sullivan designed had 14,500 psi in the gas tube while the carbine-length system had 19,000 psi. Pressure inside the bolt carrier went from 1,500 to 2,500 psi.
The significant boost in gas system pressure means that the bolt has to unlock with much more binding force on the lugs. Binding force is the friction between the bolt’s lugs and the lug abutments in the barrel extension. Too much binding force will cause bolt lugs to shear off.
This is why any AR-15 should always use the longest gas system possible for the chosen barrel length. For those of us still possessing carbine-length gas systems (or over-gassed ARs in general), there are a couple of thoughtful fixes from both SureFire and LMT to consider.
Optimized Bolt Carrier Group
The Optimized Bolt Carrier Group (OBCG) is Sullivan’s latest work and compensates for all the changes that have occurred with the AR. The OBCG features a redesigned cam pin path cut into the top of the bolt carrier, allowing the bolt to stay locked for a longer period of time. This lets pressure in the operating system drop prior to unlocking the bolt. The design is responsible for a 15-percent decrease in chamber pressure prior to unlocking the bolt, which eliminates a lot of the binding force on the lugs and significantly increases bolt life.
The OBCG has a couple of additional features designed to lengthen its travel. By lengthening the travel, the magazine has plenty of time to push a fresh round up and into the bolt’s path, giving the rifle more reliable feeding. This is especially important for those firing full-auto.
First, the gas key on top of the carrier is shorter and only uses one screw to anchor the key to the carrier. Shortening the gas key allows the bolt carrier to slide farther back into the buffer tube. Giving the carrier a longer runway takes more time to cycle, giving the magazine more time to feed the rifle. The longer carrier travel is the first of two feeding design changes Sullivan implemented on the OBCG.
The second design change is a neutrally balanced counterweight that sits between opposing springs at the back of the bolt carrier. When the bolt carrier moves to the rear and stops, the weight arrives a moment later. This greatly extends dwell time at the fully retracted position and slows down the cyclic rate enormously. For instance, a full-auto M4 drops from about 850 rounds per minute (rpm) to around 650 rpm with the OBCG in place.
The counterweight also helps when the OBCG goes into battery. The weight arrives shortly after the bolt first impacts the back of the receiver extension, much like a dead-blow hammer, and eliminates bolt bounce. Any automatic AR benefits enormously from this feature.
LMT’s Enhanced BCG
LMT’s enhanced bolt carrier, like SureFire’s, has a redesigned cam path with a similar effect. While similar in approach, LMT’s enhanced carrier came out long before SureFire’s OBCG, but neither company borrowed from the other. It’s not unusual for two smart guys to come to the same conclusion. In fact, the LMT design received kudos from Sullivan.
“Karl Lewis (LMT’s founder and owner) is a brilliant designer and machinist,” said Sullivan. Karl was flattered when I passed that along to him at this year’s SHOT Show. Lewis said with a smile, “Wow. That’s certainly high praise. I’ll try not to screw that up.”
The differences in the two carriers is LMT’s lacks the reciprocating weight and shortened gas key, but it has a bullnose on the carrier’s face that allows the bolt to extend just a bit farther from the carrier when unlocked. LMT’s carrier has the same reduction in binding force on the bolt lugs but retains the same cyclic rate as a standard carrier when fired full-auto.
LMT’s enhanced bolt is currently my favorite AR-15 bolt in the industry. It has an improved extractor and is made from a special alloy, the recipe of which LMT guards closely. Lewis once told me, “I can make a bolt for an AR-15 that I can guarantee would never break. Unfortunately, it would cost more than the rest of the rifle.” Alloys such as this are expensive and time-consuming to machine, so they always cost more than standard Carpenter 158.
When an AR-15 experiences a catastrophic failure, it is almost always the bolt that breaks. It’s either a lug on either side of the extractor or the bolt body where the cam pin passes through it. LMT’s enhanced bolt is much tougher than anything made from Carpenter 158 or 9310 steel.
In addition to the use of an excellent alloy and a better extractor, the enhanced bolt has redesigned locking lugs that also extend bolt life. A radius is cut into the top of each lug with another radius at the back of each lug where it meets the bolt body. These cuts relieve stress and prevent cracks from forming.
Enhanced carriers and bolts are a great idea for any AR-15 that is over-gassed, which means just about all of them. Anything with a carbine-length system and a barrel 11½ inches or longer are good candidates. Any AR-15 wearing a suppressor would also benefit significantly with an enhanced BCG. The only AR-15 where it may or may not help is an unsuppressed 10.3-inch barrel with a carbine-length gas system. That firearm needs a big gulp of gas to work correctly, and these carriers might not let it get enough gas to function.
Once the AR-15 community stepped away from the initial system as Sullivan designed it, we’ve been using one bandage after another. These two carrier groups from SureFire and LMT represent excellent solutions that account for the shorter gas systems and the high-pressure ammunition. If you want your rifle to last a very long time, consider buying one.
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