7.62 Thumper XCR Subsonic Semi-Automatic Rifle

XCR mini SBR with 10

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 unmistakeable 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.

Bullet comparisons - 69gr .223 Remington, 168gr .308 Winchester, 154gr 7.62x39mm, 220gr 7.62 Thumper, 240gr 7.62 Thumper

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.

Custom XCR barrels in 7.62 Thumper

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.

16 thoughts on “7.62 Thumper XCR Subsonic Semi-Automatic Rifle

  1. Landmissile

    Great write up. This is exactly what I have been looking for. Appreciate you sharing your research and findings.

    There really isn’t a heavy enough or an efficient enough bullet in the .311 diameter, I’ve been looking. Makes sense to re-barrel in .308. Seems even easier when Robinson Armament and other companies are providing readily available custom barrels.

    I have an XCR in 7.62×39. Guess I need to get in touch with RA. It would be really nice if they had an off the shelf barrel option in your 7.62 Thumper cartridge. Wishful thinking but maybe in time. I’ll be bulk loading this caliber for sure.

  2. federalist Post author

    I think Robinson is finally getting around to building .300BLK uppers, in which case they’re already machining 1:8 .308″ blanks and you would only be asking them to use a different chamber reamer. It’s an easy conversion if you already have a registered lower and 7.62 bolt. The problem with the XCR is that the piston system seems like it will only cycle subsonics out of short barrels.

    If I were starting from scratch right now I’d probably just go with .300BLK in a direct-impingement (DI) action. I haven’t tried them side-by-side but in theory DI should be quieter than a piston, which vents gas at a higher pressure. DI also cycles these light loads much more reliably: AAC reports subsonic .300 BLK will cycle the action on 16″ barrels even without a suppressor. The XCR won’t cycle without a suppressor and full gas through .25″ ports.

  3. Landmissile

    Right on. That’s good to know. Honestly I’m more interested in the larger bullet from a smaller package.

    Would it be better to stay with a 1:10 twist if I want to run it supersonic or will the 1:8 still get the job done?

    Thanks again for the information.

  4. federalist Post author

    For any .30″ barrel under 18″ I would choose a 1:8 twist, even if I never intended to shoot subsonics, because short barrels can’t get the heaviest .30″ bullets moving fast enough to reliably spin-stabilize at slower twist rates.

    Conversely, over-stabilizing bullets generally doesn’t cause problems except in the most extreme cases (e.g., some light .22″ bullets have been known to disintegrate mid-flight when super-stabilized).

    If you want to run specific scenarios the Miller Stability formula gives a good idea of what will work.

  5. Pingback: EmptorMaven » Blog Archive » Shooting products I’m looking for in 2012

  6. Landmissile

    I’ve decided to try a 14.7″ barrel with a “Mini” length gas system. I think I have it mostly sorted with RA. However, I may take you up on that. What length is the gas system?

    Are you using standard 7.62×39 dies when loading the .308 into the Russian casing or did you get custom dies for the Thumper cartridge. Seems like the 7.62×39 seating die will work well enough. Not sure the crimp die will really secure the .308 properly.

    No doubt the difference in diameter is miniscule but for the sake of safety it seems worth considering.

  7. federalist Post author

    The barrel I’m selling has the standard-length XCR gas system; I’m keeping the mini since I have plenty of other barrels for the standard upper.

    I was also worried about whether 7.62x39mm Russian dies would size and crimp the neck correctly for .308″ bullets. I’m using Lee dies and have found no problems using the same set and settings for both .308 and .310-.311 bullets: There is enough neck tension to grab .308″ bullets, and after a mild crimp I’ve never had even round-nosed .308″ bullets get pushed back by the feed ramp.

  8. Landmissile

    Ok… I just noticed the length question was already mentioned. Should have paid closer attention. I’m interested in the barrel.

    Good news in regard to the loading dies. That definitely makes it easy and a little less expensive.

  9. Steve

    Just a quick question, great info but I have a Sig 556R 7.62×39 that I was going to suppress (paperwork in) and I thought about subsonic loads, but after reading your Blog I have been educated, so will a 9 to 1 twist work? What is the bullet source for your thumper?

  10. federalist Post author

    Yes, 1:9 will be sufficient to stabilize any bullet you can buy or cast if muzzle velocity is at least 900fps. When in doubt shoot your load without the suppressor first to make sure it’s not keyholinng when it hits a paper target. Depending on the baffle design I’ve seen even unstable loads clear the suppressor without doing any damage, but it’s such a pain to get a suppressor I would play it safe!

    The problem is you’re probably getting a .311″ barrel, which really limits the bullet selection. (Good excuse to get into bullet casting ;) Remember that the heavier the bullet the more gas volume and pressure you can run and stay subsonic, and hence the more likely your rifle will cycle (which I assume would be a goal). If you’re limited to commercial bullets the heaviest .311″ I found were Lapua 200gr FMJBT, which happen to be profiled for subsonic accuracy.

    One of the reasons I went “7.62 Thumper” for this project was that there are plenty of 220-240gr .308″ bullets by Sierra and Hornady, and that extra weight can make the difference between reliable cycling and not!

  11. USMC-FURY

    Wrong! The Woodleigh .312 215gr RN is the perfect bullet, I moly coat mine.

    http://www.midwayusa.com/product/746084/woodleigh-bullets-303-british-77mm-japanese-312-diameter-215-grain-weldcore-round-nose-soft-point-box-of-50?cm_vc=wishList

    I got mine from Hungtington reloading in CA

    And if you want to shoot them at 1850 FPS Google: “Spokane gun trader 7.62×39 heavy and fast” the load data is thee for the Speer #2223 180gr RN bullets and the 215 gr woodleighs. THEY SOUND LIKE THOR’S HAMMER when the 215 gr hit at 1850 FPS!

  12. USMC-FURY

    PS steve I have the SIG 556R too, use the 2nd position on ur gas valve and you should be able to fire the 215gr without a suppressor. ps 1:9.5 twist on the SIG will stabilize the 215gr!

  13. USMC-FURY

    An excellent scope for any 7.62×39 or 300blk is the Nikon P300 as its ballistics are set up for those two and the reticle has sub sonic bullet drop built in.

    http://www.midwayusa.com/product/653571/nikon-p-300-blackout-rifle-scope-2-7x-32mm-bdc-super-sub-reticle-matte?cm_vc=wishList

    Its cheapest here:(and the Nikon scope rings for it too)

    http://www.blackbearsportinggoods.com/ProductDetails.asp?ProductCode=ELLNIK6797

    https://www.blackbearsportinggoods.com/ProductDetails.asp?ProductCode=ELLNIK835

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>