Such slam fire guns have been made with ordnance steel and featured in other youtube videos. A production model made in the USA in the 1940s is also shown.

The fascinating thing is how these open, unlocked, systems behave during firing. Personally I find it interesting that both "barrel" and "breech" recoil together even though they are not mechanically connected.

There is also a vid showing the detonation of a cartridge in an open 12g barrel. It is evident there too that the barrel recoils together with the shell. The shell is not blown out of the open barrel as one would expect after reading the "authorities".

Such things get you thinking.