SpudFiles

What makes the "bang!!" sound on a coaxle piston rifle?Is it the piston hitting the pumper or is it air gushing out the barrel at a very very fast speed.

Probably the air dumping into the barrel at an increadibly fast rate

It happens on all well made pistons not just co-ax's, if your piston is performing badly you usually don't get this gun shot sound.

On my first co-ax piston I had a rather slow setup and you could distinguish the release of the exhaust air, the piston hitting the bumper and then then loud rush of air from the barrel. They where all distinct seperate sounds. The loudest was the barrel, although the most gunshot like was from the piston hitting the bumper and the sudden loss of pressure behind the valve which caused a split second vacuum.

It's the muzzle blast, basically the air exiting the muzzle is breaking the sound barrier (though this doesn't necessarily mean your projectile is supersonic)

Make a silencer, it's easy to do, just do a search.