You can carry a machine gun or any weapon if has been demilled or is incapable of firing. However, it could still get you arrested until you can fully demonstrate to the law enforcement personel or a judge that the weapon is incapable of firing and is therfore not restriced by BATF laws. A police officer told me that when I was stopped for carrying a semi-automatic AK-47 type assualt rifle in the window of my pick-up. Doing that is perfectly legal by non-felons but it annoys police officers.

That might be a good option to reenactors that do not entend to fire their weapons. You could posses two muzzle loading weapons, one for hunting and target shooting and another one that is incapable of firing and use it just for reenactor events.

Or, just render your reenactor type weapon incapable of firing and don't shoot at the events.