Sorry to say, your #16 sheet-metal plus a couple 2x6's on either side with bolts is not heavy enough nor strong enough to consistently and repeatedly do the job on the scale required. 16guage is about as rigid as a wet noodle. I would go with 1/8", 3/16" or even 1/4inch sheet metal.
However, please, do NOT let my critique discourage you from building and using this demonstrator repeatedly until you can crowd-source the necessary repairs and upgrades.
Personally, I don't care if having their vertebra repeatedly used to sound the flimsiest gong possible makes a few billionaires last moments that much more painful and humiliating.
It is worth mentioning though, the French Revolution did not have access to semi-auto, reliable, firearms, while both bullets and guns are, today, both plentiful and cheap.