![]() Halfway through my development of this rifle the Freedom Group announced its own solution to the same objective: the now SAAMI-standard. 308″ bore with a 1:8 twist rate, and it’s best fired with. So a 7.62 Thumper gun is 7.62x39mm chamber but a. 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. 30-caliber bullets heavier than 200gr are. 311″ diameter, whereas the only widely-available. And this is why we make the distinction between 7.62 Thumper and 7.62x39mm: Russian bullets are. 30-caliber barrel has such a fast twist a new barrel is going to be part of any subsonic conversion. 30-caliber bullets are 240gr, which require a barrel rifled with a 1:8 twist to stabilize at subsonic velocities. 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. Peter Cronhelm posted a fair amount of research on subsonic shooting with the 7.62x39mm from bolt guns. 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. ![]() However various drawbacks have prevented them from being widely adopted. ![]() A host of proprietary and wildcat cartridges have existed for this purpose for decades, like the. 30-caliber bore.ħ.62 Thumper is one of a number of specifications for short rifle cartridges designed to shoot. Before long you will realize that if you’re going to build a subsonic rifle capable of producing appreciably more energy than a. And longer bullets combined with slower muzzle velocities require faster barrel rifling to get spin-stabilized enough to shoot straight. But as I explained in a previous post the only practical way to make a bullet heavier is to make it longer. To solve these problems you soon realize that what you want is to shoot a much heavier bullet. 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. 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. The only way to further suppress a rifle’s noise is to shoot the bullet below the speed of sound. 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. Like most, I began my quest for subsonic rifles shortly after buying my first rifle suppressor (a.k.a. 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. 357 magnum at point blank range, and greater than a. These leave the muzzle at 1000fps, which means they carry 500 foot-pounds of energy - comparable to a. In the following video I shoot a ten-round magazine of subsonic 220gr bullets. ![]() 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.” Shown is the “mini” upper with a 10″ barrel and AAC’s Cyclone suppressor. This is an XCR semi-automatic rifle in 7.62 Thumper.
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