A 12 gauge shell is a nice fit in a 3/4" steel water pipe.
a 3/4" steel pipe is a nice "sliding fit" inside a 1" steel pipe.
This one struck me funny.....
https://youtu.be/ZG6Y5TIDJMg
The gun [below] is a little more practical, but there is no way to keep the shell seated in the barrel if you point the gun up. Extracting the shell might be tough without a too lif it is swollen.
I think the Philipinos get credit for making these..?
https://youtu.be/xt3NuCbxSQQ