This is found mainly on autocockers, but now other guns like spyders and piranhas have adapted them. This part stops players from thumping their guns. This is an attempt to increase the speed of their autocockers. The beaver tail stops this from happening. The part of the gun that strikes the gas valve to release a jet of gas and push the paintball out of the gun.
Bolts move back and forth in their action to fire, re-cock, and then fire again.
It is the air tank. Common sizes are 7oz, 9oz, 12oz, 14oz, 16oz, 20oz, and 24oz Co2 tanks. Common nitrogen systems are 48Cu, 68Cu, and 114Cu. The pressure could be 3000 or 4500psi. This refers to someone getting hit with a paintball, but it did not break on them.