As a crude guess, I would suggest a 1" piece of plate steel. That should
stop anything a 20mm cannon can dish out save for maybe a hardened ball. To
be on the safe side, I would gradually work up to full load (and ball
harness) by shooting with reduced charges and projectiles of softer texture.
Be esp. aware of the bounceback tendency of hard projectiles.

Bob Swinney

PS: If you get disgusted with the project, sent me the workpiece and I'll
take it from there!


"steamer" wrote in message
...
--I'm trying to figure out what might serve as an effective
backstop for my 20mm cannon. Strictly black powder and muzzle loading mind
you, but what would it take to stop something coming out of that barrel
so's I don't scare the bajeezus out of the neighbors?

--
"Steamboat Ed" Haas : Quando Omni
Hacking the Trailing Edge! : Flunkus Moritati
http://www.nmpproducts.com/intro.htm
---Decks a-wash in a sea of words---