Design
The typical recoilless gun functions very much like a conventional gun. The projectile and propellant are supplied as a single round and loaded into the breech. When fired, however, instead of all the propellant blast following the projectile out the barrel, a large portion is allowed to escape to the rear, gaining a rearward directed momentum which is nearly equal to the forward momentum of the projectile. This balance of momenta ensure that the momentum of the rifle/projectile/exhaust gas system is conserved without imparting much momentum (recoil) to the rifle itself. Since recoil has been mostly removed, the heavy and complex gun carriage and recoil damping mechanism can be dispensed with. Despite the name, it is rare for the forces to completely balance, and real world recoilless rifles do recoil noticeably (with varying degrees of severity). Recoilless rifles are maintenance-intensive weapons, and if the breech and gas ports are old, damaged, plugged or poorly maintained, the recoil-damping effect can be reduced or lost altogether, leading to dangerously powerful recoil. Conversely, if a projectile becomes lodged in the barrel for any reason, the entire weapon will recoil forward, in the manner of a rocket.
Unlike a rocket launcher, which fires fin-stabilized rockets from a smooth bore, recoilless rifle rounds resemble conventional artillery shells. They generally have a pre-engraved rifling band to engage the rifled launch tube, spin-stabilizing the projectile, hence the term "rifle". The "case" area of the shell can be perforated to vent the propellant gases which are then directed to the rear, as the base of the shell disintegrates.
Since venting hot gases to the rear can be dangerous in confined spaces, some recoilless guns such as the Armbrust and MATADOR use a combination of a countershot, smoothbore barrel and pistons to avoid both recoil and back blast. The fin stabilized Armbrust "cartridge" contains the propellant charge between two pistons with the warhead in front of one, facing forward, and an equal countermass of shredded plastic in front of the other piston. Upon firing, the propellant expands rapidly pushing the pistons outward. This pushes the projectile forwards towards the target and the countermass backwards providing the recoilless effect. The shredded plastic countermass is quickly slowed by air resistance and is harmless at a distance more than a few feet from the breech. The pistons jam at the ends of the barrel trapping the hot propellant gases inside. All this allows safe firing in enclosed spaces.
Read more about this topic: Recoilless Rifle
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