But is there a difference in the strength requirements between a noise making cannon and the real thing? It would seem that if I am just going to be using an open ended barrel, that I wouldn't need something that could withstand 70,000 psi. Does anyone know how the chamber pressures compare between firing a blank shot and firing a loaded cannon? I don't want to scrimp on materials and make something unsafe, but I also don't want to invest in something that I will never use to fire projectiles.
Your questions hit pretty much the salient points on this subject. The difference between a noise making cannon and a shot launching cannon is the operator. Unless you can guarantee that it will never ever be used to fire a projectile, it is an accident waiting to happen when someone does so. Maybe not you, maybe not while you own it, but unless it is destroyed when you cease to use it, the problem remains.
Technically, one of the problems of backyard/home shop cannon making is the lack of engineering put into them. The chamber pressures are not known, so people take that as an excuse to do no calculations and use poor designs. Normally the maximum charge for a projectile launching shot is about half that of a blank shot. The extra powder in a blank shot becomes the "projectile" so that pressure is raised to the point that the report is a boom instead of a whoosh. So there is some pressure developed even though we don't know the actual number.
The use of 70,000 psi material does not mean the actual stresses in the material are allowed to reach 70,000 psi. Normally you limit the actual stesses to about half the maximum stress to prevent fatigue failure over a number of shots. But building to the absolute minimum of material is not something that should be done without actually doing the math, and that requires tested chamber pressure figures, and still doesn't address the issue of the barrel eventually being used outside the design criteria by launching shot.
The "literature" is filled with reports of people injured or killed by explosions of defectively designed and poorly made "cannons" so the risk of such a device is very high. Many times the guns have been fired more than once without accident before they finally let go. I strongly suggest you stay away from an inadequate design.
