Check out this website, especially if you live in the UK… John Webster, a reenactor and builder of LPG gas guns, offers a great alternative to using blank guns. WTAC – Replica and Gas Firing Weapons for Re-enactors and Films. He builds some pretty impressive non-functioning gas weapons for reenactments, as well as tanks and other military vehicles.
“…These are gun fire simulators for use by WW2 re-enactors. They are built to sound like live guns but cannot fire projectiles and do not use blanks. They can be owned without any form of license in the UK and can be used in battle re-enactment at about 10% of the cost of using traditional blank fire replicas which cost more to buy and use. They are fired by using a mixture of propane and oxygen with electonics and valves built into the replica to control firing.”