Surefire Optimized Bolt Carrier Assembly
July 20, 2018
Jim Sullivan is 85 years old and still drives like a bat out of hell. For those unfamiliar with the name, Jim Sullivan designed the AR-15 60 years ago.
Eugene Stoner is most commonly credited for the AR-15 but that isn't entirely accurate. Stoner designed the AR-10 (he was a .30-caliber man) with Sullivan as his assistant. Sullivan's input on the AR-10 is why the cam pin is shaped the way it is and why the gas system sits on top of the barrel instead of on the side. The AR-15 was left almost entirely to Jim Sullivan and the project helped set him on the path of miniaturizing popular rifle designs. (His first was the AR-15 and his second was Ruger's Mini-14.)
I recently had the opportunity to make the drive from Prescott, Arizona, to Gunsite with Sullivan at the wheel, availing me the opportunity to ask the man all the questions that I've accumulated over the years. Most of those questions were centered around his opinion of what's become of his creation.
Sullivan never anticipated the rifle becoming so popular. Love it or hate it, the AR-15 has spawned an industry all its own. Most of what's happened to the rifle since its inception has taken it far from the initial design parameters.
The AR's Evolution
One of the first major changes to the M16 was to shorten the barrel and the gas system that sat on it. Moving the gas port closer to the chamber meant the rifle started the extraction process much earlier than Sullivan intended. "Colt had an excellent engineer at the time that came up with a very simple and fairly effective solution. He made the buffer heavy," Sullivan elaborated.
The heaver buffer in what's now known as the "M4" holds the bolt and bolt carrier in place longer, allowing pressures in the rifle to drop before extraction begins. This helps prolong bolt life, but doesn't quite eliminate the problems that come with the shorter gas system.
The next change comes from the Special Operations community and their desire for shorter rifles for urban combat. Barrels were shortened even further to 10.3 inches but designers learned they had to enlarge the gas port to send enough gas back to the receiver to reliably cycle the bolt.
Hot on the heels of the 10.3-inch barrel was the addition of a suppressor. Rifles with short barrels have tremendous exit pressure (the pressure in the barrel when the bullet exits the muzzle) and are extraordinarily loud.
Sadly, adding a suppressor to a 10.3-inch barreled AR that wasn't designed for use with one creates some significant problems. The short barrel has very little distance between the gas port that feeds the action and allows it to cycle and the muzzle. Once the bullet exits the muzzle there is no longer pressure to cycle the action. Putting a suppressor on that same short barrel now means the gas system stays pressurized for much longer. That causes the bolt to cycle much faster than it was ever meant to, which shortens bolt life.
The final change that is so horrific that it boggles the mind is the military's use of M855A1 ammunition that's loaded hot enough to have a chamber pressure of 64,000 pounds per square inch (psi). The AR-pattern rifle was designed for ammunition with a chamber pressure of 50,000 psi. The higher pressure combined with a short gas system greatly shortens bolt life and represents one of the worst decisions the military has made in quite some time.
Surefire, a company known for high-quality illumination devices and suppressors, and holder of the suppressor contract for the U.S. Special Operations Command (SOCOM), has been quietly observing the many changes the military has made to both rifle and ammunition and thought it was high time for a sanity check.
Surefire reached out to Sullivan and asked him to take a look at what's happened recently to the AR in the hopes that he could mitigate many of the stresses that the changes have had on his rifle. Up until now, few, if any, have taken a comprehensive look at all of the changes and their overall effect on the AR.
A new, enhanced bolt carrier group (BCG) was Sullivan's solution created for Surefire to remedy the decades of mistreatment his rifle has received. The bolt is made from standard Carpenter 158 case-hardened steel, just like the military requires. However, the real changes are found with the bolt carrier.
Sullivan reshaped the cam pin slot to allow the bolt carrier to move significantly more rearward before twisting the bolt to unlock it from the barrel extension. This small delay keeps the action closed longer and allows pressure inside the chamber and bore to drop significantly. Lower internal pressure means there is less binding force on the bolt lugs when it does unlock. Sullivan's solution is essential to preserve bolt life since the military uses ammunition with a chamber pressure of 64,000 psi.
Sullivan also shortened the gas key on top of the carrier. Instead of a longer key with two screws, his shortened key has only one screw. This allows the bolt carrier additional rearward movement without contacting the lower receiver. The additional length of travel in the bolt carrier greatly slows down the cyclic rate when firing full auto, but gives the magazine spring enough time to push the ammunition column up in front of the bolt, even if it is an old and tired spring.
The reciprocating weight in the back end of the bolt carrier is one of the most ingenious features. The rearward movement of the bolt carrier pushes the spring-loaded weight all the way to one end of its range of motion. When the carrier starts moving forward, the weight moves to the other end of its range of motion. The time it takes the weight to make that transition is the amount of time the carrier is held to the rear. The delay from this reciprocating weight further slows the cyclic rate of an AR-15/M4/M16, which allows the magazine to feed more reliably.
We live in a day and age where the changes the military makes to its rifle and ammunition are done in a vacuum. First, Big Army wanted a carbine version of the M16 and along comes the M4. Next, SOCOM wanted a 10.3-inch barrel to make close-quarters battle easier. That made the gun loud. So, every rifle got a suppressor. Finally, the U.S Army jacked chamber pressure through the roof in pursuit of better terminal ballistics, which they got at the expense of rifle durability. None of those changes were done with any thought of what might come next.
I think it's brilliant that Surefire, after witnessing all of the above, had the sense to reach out to the guy that designed the rifle in the first place to get his input on what has happened to it and how to solve the second- and third-order effects these modifications have induced.
Jim Sullivan might be a little long in the tooth, but he's still smart enough to come up with a low-cost, drop-in part that fixes several decades of tinkering from amateurs. Tip of the hat to you, Jim.