When Speed, Peter and Billy are being gassed, the Mastermind says that it is sarin gas that is being used. Even at low concentrations sarin gas can be fatal in as little as one minute and without the administration of an antidote will lead to death usually due to asphyxiation. No antidote is ever shown to be administered to any of the characters but they are fine with seeming no lasting effects as soon as the gas is ventilated. Clothing that is exposed to sarin gas can continue to expel the toxin for up to half an hour causing constant exposure, yet this does not seem to affect any of them either.
Every time the shotgun is cocked, nothing is ejected.
The motorized rifle, mounted in the room, doesn't have any kind of mechanism attached to the trigger to actually be able to fire. The muzzle flashes are obvious CGI.