A common rifle for military and police are gas operated firearms. These include, but are not limited to, AR10, AK-47, AK-74, M14 M16, M16A2, M4, FN SCAR family, M110, MK 11, and others. These gas operated rifles have been produced by numerous manufacturers. These weapons, typically shoot, but are not limited to, 5.45 mm, 5.56 mm, 6.8 mm, and 7.62 mm bullets which provide very high bullet velocities.
These gas operated gas operated-style rifles utilize either a direct gas impingement system or a gas and push rod system for operating their ejection and loading mechanisms, in an automatic mode and a semi-automatic mode. The expanding gas from the cartridge propellant is tapped from a port in the barrel intermediate the chamber and the muzzle end of the barrel. In the direct gas impingement system, a conduit extends from the port to the upper receiver and into the region of the bolt carrier. In the gas and pushrod system, the gas impinges against the push rod which extends to the upper receiver and into the region of the bolt carrier. During the initial firing of the cartridge, the bolt is locked into the barrel extension, the gas forces the bolt carrier backward a short distance to unlock the bolt. As the bolt carrier moves toward the butt of the gun, a bolt cam pin, forces the bolt to rotate, by this time the bullet has left the barrel. The inertia of the bolt and bolt carrier continues the rearward motion causing the bolt to extract the fired empty cartridge. A spring absorbs the rearward motion of the bolt and bolt carrier forcing the bolt and bolt carrier forward to engage the next cartridge in the magazine and push same into the chamber ready for firing.
The gas pressures for operating the gas operated style weapons are significant and with the 5.56 mm cartridges the exit velocities, typically in excess of 2700 fps, way exceed the sound barrier (about 1,126 fps). Associated with these velocities are high bullet travel distances, in excess of 2 miles, and noise levels, including from the bullet breaking the sound barrier that cannot be effectively suppressed.
Modifications have been developed for these gas operated weapons to shoot low mass rounds at low velocities that utilize telescoping cartridges-practice ammunition. Typically the cartridges have very low mass, compared to lethal rounds, and may also have frangible projectiles with marking media. The modifications include a bolt and bolt carrier modification that allows the bolt to retract entirely by the propulsion of the expanding telescoping cartridge with no assist from the gas port, effectively changing the function of the weapon from a direct gas impingement system to a direct blowback system. The bolt does not lock into place rearward of the chamber. The energetics in these cartridges is minimal compared to a normal lethal round. For certain manufacturers, the primer contains the entire energetic load for launching the projectile and operating the ejection mechanism by the force from the telescoping cartridge. See Force on Force™ ammunition available from Federal Cartridge Company, the owner of the instant invention. See also U.S. Pat. Nos. 6,931,978; 6,178,889; 6,564,719; 6,625,916; 6,439,123; 5,677,505; 5,492,063; 5,359,937; 6,625,916; 7,278,358; 8,146,505; 7,225,741; 7,621,208; 7,984,668; U.S. Publ. Nos. 2010/0269724.
In certain instances, it would be desirable to have lethal cartridges that may effectively operate the gas operated style ejection mechanism and that travel below the speed of sound. Such cartridges could effectively be mixed with normal high velocity lethal rounds without gun modification. The lower velocity rounds could then be effectively used with suppressors. Attempts to manufacture such ammunition have not yet been entirely successful. For example, simply lowering the amount of propellant to reduce the exit velocity of the bullets results in less gas pressure to reliably operate the weapon. Also, using lower amounts of propellant results in lower ignition pressures, which in turn results in very dirty propellant burn and/or incomplete combustion which causes gun malfunctions. Although weapon modifications are possible to utilize less powerful ammunition, then switching back to the regular full energy ammunition is problematic and certainly cannot reasonably done in a combat situation.