I think you could make one that would be safe if you are careful and use good materials and processes. You'd probably want to assume you could have at least 25,000 psi chamber pressure, then use a safety factor of at least 1.5. some would say that's too low, so don't take my word for it, I'm no expert, just calling attention to something you need to calculate. Whatever plug you use and the straps and strap trunnions have to resist the instantaneous forces generated by that chamber pressure pushing on the breechblock. If you have only one square inch of area at the front of the breechblock exposed to that pressure, an instantaneous force of 25,000 pounds is generated that's trying to push the breechblock out of the gun. When you look at these large forces, it isn't hard to figure out why strap guns were dropped as soon as people figured out how to contain the pressure within the chamber.
