This is an XCR semi-automatic rifle in 7.62 Thumper. Shown is the “mini” upper with a 10″ barrel and AAC’s Cyclone suppressor. This is an awesome firearm that offers the power and accuracy of a rifle with the option of using subsonic loads that are not only hearing-safe but which “won’t wake the neighbors.”
In the following video I shoot a ten-round magazine of subsonic 220gr bullets. These leave the muzzle at 1000fps, which means they carry 500 foot-pounds of energy — comparable to a .357 magnum at point blank range, and greater than a .45 ACP pistol. And because they are very long, ballistically efficient rifle bullets they retain 80% of their energy out to 300 yards, which is roughly the outer limit of being able to accurately place a subsonic bullet.*
Like most, I began my quest for subsonic rifles shortly after buying my first rifle suppressor (a.k.a. silencer). After all, it was cool to be able to shoot without hearing protection, but supersonic bullets make a loud and unmistakable sonic crack of their own. The only way to further suppress a rifle’s noise is to shoot the bullet below the speed of sound.
What is 7.62 Thumper?
In principle it might seem easy to slow down a bullet: just put less powder behind it, right? However a number of undesirable things begin to happen as you do this with a given cartridge: First, as you continue to reduce the powder charge below roughly 80% you will begin to get increasingly inconsistent muzzle velocities, which dramatically reduces the gun’s accuracy. Drop the charge even further and you occasionally get a bullet stuck in the barrel, sometimes accompanied by a potentially catastrophic phenomenon often called “secondary explosive effect” which has destroyed many guns! Also, since a bullet’s energy equals mass times velocity squared you will be severely weakening your bullet’s power as you slow it down. To solve these problems you soon realize that what you want is to shoot a much heavier bullet. But as I explained in a previous post the only practical way to make a bullet heavier is to make it longer. And longer bullets combined with slower muzzle velocities require faster barrel rifling to get spin-stabilized enough to shoot straight. Before long you will realize that if you’re going to build a subsonic rifle capable of producing appreciably more energy than a .22LR it’s going to need at least a .30-caliber bore.
7.62 Thumper is one of a number of specifications for short rifle cartridges designed to shoot .30″ bullets at subsonic velocities. A host of proprietary and wildcat cartridges have existed for this purpose for decades, like the .300 Whisper and .300-221. However various drawbacks have prevented them from being widely adopted. The attraction of 7.62 Thumper is that it uses a standardized and widely-available case and chamber: 7.62x39mm Russian, which is the standard caliber for AK-47s. Peter Cronhelm posted a fair amount of research on subsonic shooting with the 7.62x39mm from bolt guns. My goal was to start with that and work up a gun and load that would shoot standard 7.62x39mm rounds but also reliably cycle subsonic rounds in a semi-automatic rifle.
The heaviest standard .30-caliber bullets are 240gr, which require a barrel rifled with a 1:8 twist to stabilize at subsonic velocities. Since no standard .30-caliber barrel has such a fast twist a new barrel is going to be part of any subsonic conversion. And this is why we make the distinction between 7.62 Thumper and 7.62x39mm: Russian bullets are .311″ diameter, whereas the only widely-available .30-caliber bullets heavier than 200gr are .308″. In theory you can shoot .311″ and .308″ bullets in either bore diameter, but I have tried that and the effects tend to be either bad accuracy in the case of undersized bullets or else shredded jackets and exploding bullets in the case of Wolf Military ammo in the high-twist .308″ bore. So a 7.62 Thumper gun is 7.62x39mm chamber but a .308″ bore with a 1:8 twist rate, and it’s best fired with .308″ bullets.
What about .300 AAC Blackout?
Halfway through my development of this rifle the Freedom Group announced its own solution to the same objective: the now SAAMI-standard .300 BLK caliber, which is designed specifically to work on the AR-15 platform. The ballistics are virtually identical to 7.62x39mm, and as with 7.62 Thumper a standard rifle only requires a new 1:8-twist .308″ barrel to shoot accurate subsonic loads. The advantages of .300 BLK are (1) It uses the small .223Rem bolt standard on AR-15 rifles instead of the large 7.62x39mm bolt standard on AK-47s, and (2) Remington will be producing factory subsonic ammunition, whereas you still have to load your own 7.62 Thumper ammo.
Why the XCR?
Getting a 7.62x39mm to shoot 220gr subsonic would be easy enough. But getting it to cycle a semi-auto action was unknown territory. I knew that I wanted a piston-driven semi-auto with an easily-modified gas system.
Given that I wanted to work with 7.62x39mm AR-15s with their small bolt were immediately ruled out. I also knew that I would potentially be yanking and changing a lot of barrels to get this to work. Fortunately, my favorite tactical rifle, the XCR, met the bill. The XCR’s barrel goes in and out with a single screw. The XCR also has one of the best piston gas systems for tuning: From the factory it comes with 5 hand-adjustable settings. And if those don’t work it’s easy to remove the regulator and gas block to enlarge holes to increase pressure.
The machinists at Robinson Armament, maker of the XCR, were also willing to build some custom barrels for this project at a reasonable price. I sent them a Shilen stainless steel barrel blank bored to .308″ with 1:8 twist cut rifling. RA cut it in half and put it on their machines to turn out the two drop-in short barrels shown here: 10″ for the shorter “mini” XCR upper, and 12″ for the standard-length upper.
Short barrels and subsonics go hand in hand. Longer barrels serve only to increase a bullet’s velocity. Of course you have to register a rifle as an SBR with the ATF before you can install a barrel shorter than 16″, but this entire project is only interesting if you are already in the practice of registering NFA items like the suppressor you’re going to put on the end of the barrel.
The Subsonic Load
With the rifle in hand my goal was to work up a load that would shoot right at 1000fps — about .9Mach under comfortable atmospheric conditions, and safely below the transonic barrier where bullets start to make their own flight noise. But I also wanted one that would do so with enough energy to cycle a semi-automatic rifle action, and this has not been easily done in the past! It is compounded by the fact that no smokeless powders commercially available are designed for short-barrelled rifles. The military has specified and bought batches of powder customized for SBR loads, but they don’t leave any for sale to the public. For now reloaders are stuck with suboptimal options in terms of bulk and burning speed. The new standby for subsonic loads, TrailBoss, is too bulky to work in any rifle cartridge capable of firing and cycling both high-velocity and subsonic ammo. After significant research and testing I have found that the two best powders for subsonic SBRs are IMR 4227 and IMR SR-4759. In the case of the 7.62 Thumper XCR SBR, 12 grains of 4227 or 11 grains of SR-4759 over 220gr-240gr bullets will shoot right about 1000fps and, with a suppressor, provide enough pressure to reliably cycle the action.
* Regarding Maximum Effective Range of Subsonic Bullets: The biggest constraint on subsonic bullet range is encapsulated in a concept called “Danger Space,” which can be defined as the maximum error in range to a target that will still result in a hit. It is a function of both target size and the bullet’s fall angle at the target’s range. At subsonic velocities the latter factor quickly becomes overwhelming: For an 8″ target at 300 yards danger space is 30 yards: I.e., if there are no other sources of ballistic error you can only tolerate a ranging error of +/-15 yards and still expect to hit the target. Any more and you will either overshoot or fall short.
Shooting targets of known range this is still feasible: I tested this gun against 8″ steel plates at exactly 300 yards. The shot requires 40MOA of elevation, and the bullet takes a full second just to reach the target, but we made a first shot hit and were consistently ringing the steel. However when you introduce the uncertainties in range and other ballistic effects one might encounter in “real-world” situations like hunting this would become a difficult shot.