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BerdanIII · May 31, 2008, 03:58 PM

Submachine guns (MP5, Sten, Sterling) use a variation of the same blow-back principle; the inertia of the bolt at rest is overcome by the back pressure created by firing the cartridge and moves rearward, extracting and ejecting the empty case on the way and chambering the next round on the return trip, powered by the recoil spring. No gas or gas system is involved. Any sub- or supersonic load that can overcome the bolt inertia (and recoil spring pressure) will function the action. For gas-operated systems (M16, M4, AK47, AK74) functioning is dependent on the pressure of the gas at the gas port being high enough to push the operating rod or piston back with enough force to push the bolt back, unlock the action and extract and eject the fired case. The next round is chambered by the energy stored in the operating spring that was compressed when the bolt was pushed back. For automatic functioning to occur with subsonic ammo, it has to produce the same pressure as supersonic ammo, which is not easily done. The Russians solved the problem by using a high-pressure chamber in the suppressor that creates enough back pressure to cycle the action. They also use a 180-gr. bullet in their subsonic 7.62x39mm ammo which helps maintain gas pressure in the barrel, being heavier and more difficult to move than the normal 125-gr. bullet. The heavier bullet gives better terminal performance, too. They fielded a 9.3x39mm round and a suppressed sniper rifle that was designed from the ground up for that purpose alone. You could also go with a slower-burning powder that would reach the required pressure just as the bullet was passing the gas port, but that would really involve some finagling. I was recently on hand for some suppressed M4 and bolt-action .308 WCF firing, and even with supersonic ammunition, muzzle blast was on par with a .22 Magnum rifle.

May 31, 2008, 03:58 PM	#3
BerdanIII Junior Member Join Date: May 6, 2008 Posts: 9	Different loads for silenced weapons Submachine guns (MP5, Sten, Sterling) use a variation of the same blow-back principle; the inertia of the bolt at rest is overcome by the back pressure created by firing the cartridge and moves rearward, extracting and ejecting the empty case on the way and chambering the next round on the return trip, powered by the recoil spring. No gas or gas system is involved. Any sub- or supersonic load that can overcome the bolt inertia (and recoil spring pressure) will function the action. For gas-operated systems (M16, M4, AK47, AK74) functioning is dependent on the pressure of the gas at the gas port being high enough to push the operating rod or piston back with enough force to push the bolt back, unlock the action and extract and eject the fired case. The next round is chambered by the energy stored in the operating spring that was compressed when the bolt was pushed back. For automatic functioning to occur with subsonic ammo, it has to produce the same pressure as supersonic ammo, which is not easily done. The Russians solved the problem by using a high-pressure chamber in the suppressor that creates enough back pressure to cycle the action. They also use a 180-gr. bullet in their subsonic 7.62x39mm ammo which helps maintain gas pressure in the barrel, being heavier and more difficult to move than the normal 125-gr. bullet. The heavier bullet gives better terminal performance, too. They fielded a 9.3x39mm round and a suppressed sniper rifle that was designed from the ground up for that purpose alone. You could also go with a slower-burning powder that would reach the required pressure just as the bullet was passing the gas port, but that would really involve some finagling. I was recently on hand for some suppressed M4 and bolt-action .308 WCF firing, and even with supersonic ammunition, muzzle blast was on par with a .22 Magnum rifle.