Hi Tamid, I've had additional time to think about your project.
Since you are already invested in your system, any of the suggestions to do something else would seem pointless.
Only a crazy idiot would install crude debilitating explosives in a home firearms display. You could probably share a cell after the neighbor kid gets maimed.
I like the barrel lock design. Hidden, cumbersome, and effective. After all, a shotgun is it's barrels. This is how I would manufacture the snap cap locks.
I would cut a 6mm wide and 3mm deep key slot 80 mm down the shaft from its tip. At the 60mm mark I would turn a continuous groove 6mm wide and 3mm deep around the shaft.
I would index the rod to some arbitrary degree, 45, 60, 90 or other, and cut another key slot back toward the breech end, starting at 70mm, and ending at 30mm.
The locking cartridge would act like the bolt of a rifle.
I would turn a sleeve of chamber size od, and bar size +1mm id, and then thread a 5mm set screw through the sleeve at 75mm from a turned rim/base. Protruding 2mm into the sleeve.
Now you have a lock, not unlike a rifle's bolt.
Push the cartridge onto the rod, turn it X degrees, and then pull it back to you. You'd destroy the barrels before anyone figured out that to remove the barrels, first you must push forward 20mm, rotate X degrees, and then pull the cartridge out.
The machining is very simple, you could choose any number of "keys" for turning the cartridge, and it would be innocuous when installed.
The force to pull the barrels off the rods and chokes would be considerable. Make sets in all the gauges if you want.
What took some time to describe, is very simple machining work.
