Any chance of a photo?

Are you just trying to make the barrel itself, or the entire cannon?

If it's just the barrel, do you think it would be easier to make an original mold on your own, rather than actually duplicating the concrete barrel?

Sue

Sounds like a two-piece urethane rubber mold or possibly a "brush-on" rubber mold and rigid mother mold might be in order for a project of that size. Urethane will give you infinite detail and still last through a couple of hundred concrete pours if you use a high shore (density & abrasion resistance) number. Check out Reynolds Advanced Materials. They carry a full range of mold making materials and are very helpful folks. Best of luck...sounds interesting.

Here is a link that might be useful: Mold Making Materials, Goodies and Info

I don't have a picture, but it's just the barrel. I was wondering how to get the rounded side of the barrel? If I lay it in a box and pour rubber over it, the bottom will have some area that's not covered. Right? If I use brush on rubber, when I peel it off, how do I reinforce it so that when I pour concrete into it, it won't bend? Then I guess I would have to make a 2 piece box that I can press together? Or do I pour the concrete into 2 halves and then join them? If so, how do I join them? I would guess when the concrete is still kind of green? I have no Idea.

Nat
You can make a two piece mold of the cannon laying down top/bottom but then stand it on end to cast.
You might try to use a large waterproofed Sonotube and cut it in half lengthwise and cast your cannon in it. You need plywood half round cutouts to support the half tube while making your mold.
I use Portland and cryogenic perlite for making molds.

Check out the Smooth-On website. Tons of good basic information.

Here is a link that might be useful: Mold Making & Casting Info...