Got bored.  How much powder for 36cal ball? No idea on the steel, but low carbon. Should I trash it and forget I made it?

In high school I made something similar. I put about a teaspoon of black in it from unrolled BlackCats. I had it pointed into the ground and when it went off it launched into the neighbors yard and almost hit a baby that was sleeping at the edge of the pool on a lounge chair. Bottom line. Be damned careful.

User name is appropriate.

As a fellow that has a Winchester Signal cannon, I must say, I like the profile of your cannon.  I do not know the dimensions or scale of your project, but, i do wonder if the touch hole is as large as it looks or is it an illusion?  The ones' friends have built had a touch hole just large enough for some cannon fuse to fit into.  If memory serves me that would be 1/16-1/8".

Nothing like big bangs on the 4th of July!  The smoke from black powder only adds to the joy and the fog of wars past.

.36 cal ball, about 15 to 18 grains FFFg. I made a .50 out of brass years ago but just stuffed it with wadding and no ball, hell of a noise maker.

Can't really tell the size of it in the pictures, but if that's a .36, I'm guessing the actual bore length is maybe 2" or so? Being such a short barrel length, you won't need much powder. Even a .36 squirrel rifle can be fired with a small charge.

It's a simple matter to proof test it. If you can get 20 grains in there and still have room for the ball on top, I'd try that. Use a patch around the ball (as you would with a rifle) rather than a bare ball with wadding on top, the point being to generate the most pressure that barrel is ever likely to see. Find some way to secure it before firing so if the worst happens, no one gets hurt or anything damaged. Maybe rig up something made from 2x4s (as long as one side is open; don't seal it inside) or something you can use to contain it for testing purposes. Maybe put it inside an old tire. Something that will stop parts from flying if it blows (or let them fly in a direction of your choosing). Use a fuse rather than applying a heat source to the touch hole, so you don't have to be there when it fires. Put it behind something like a large tree or a shed, light the fuse, then leave the area (don't stay there and watch). If it survives, you should be fine to mount it on a tiller and shoot it with 10 grains of powder from then on.

I agree about the depression for holding powder being too deep. Making it too deep can create a weak spot in the barrel. My .32 gonne doesn't really have a depression at all, just beveled edges around the touch hole. My .36 gonne does have a depression for extra powder, but it's pretty shallow. Same thing will all my bigger gonnes. It's only meant to hold a tiny bit of extra powder. You don't want to make it too large and deep and compromise the integrity of the barrel.

Put it on a shovel handle.  Give yourself some distance.  Keep the power charge low.

I used a center drill to taper it down to the hole

Yea it’s about two inches