Soldiers can't sue the government. That doesn't say much about re-enactors, tho'. If I were the lawyer for a group, I would be advising my Directors to do what is necessary to avoid causing burns and injuries to the members, by providing, or requiring flash deflectors on the guns used in the volley fire demonstrations.
What happens if someone faints in the heat, and falls into the live of fire, and takes the gases in the face, or eyes? Or stumbles to arrive at the same place at the wrong time? Do you think, " Gosh, we were only shooting blanks", is going to play well with a jury that is being asked to find negligence, and award large damages to some one who has lost an eye??