The majority of weapons used at a mainstream reenactment are single shot muzzle loading rifles. Typically the units are formed up prior to the battle and the weapons inspected.
Rifles have the ramrod dropped down the barrel to insure it's not loaded & it's clean and then the trigger & lock are given a function check. During most “battles” the rammer is not used and the powder is simply poured down the barrel and set off by the cap.
Pistols are generally only used by infantry officers (not much, they have other things to think about) and the cavalry. The cylinder has a small powder charge and cap but the remaining void is filled with something like cream of wheat.
In this area the “Officers” in charge of such units have a greater responsibility to insure the men under them have empty & clean weapons prior to loading with blank charges.
While an accident it is one that could have been prevented. I'll bet the units on the field have a real good idea down to a couple just what unit and individual did this.
Isn't some sort of wadding needed to both hold the powder in place prior to ignition, and to maintain pressure until the powder has substantially ignited?