You say the chamber thickness outside is 3 times the chamber diameter...
Outside diameter of the chamber piece needs to be three times the inside diameter, for example, a 1" bore chamber would need to be 3" outside diameter. This makes the wall thickness equal to the inside diameter.
Or could I have a chamber machined that would fit in a thick DOM barrel?
This would be possible and the DOM would not have to be especially thick as the chamber piece would be taking the maximum pressure. You could thread the outside of the chamber piece and the inside of the DOM and screw them together. However, DOM is not the best choice for a barrel as all the DOM I have found has a welded seam which has the potential for fouling to penetrate the seam and cause corrosion. A better choice than DOM is seamless tubing. Its downside is the the bore is not quite so round as DOM.
Another downside of this style of construction is that your cannon is going to be cylindrical instead of the typically taper shape of most cannon barrels. This usually does not concern the first time cannon builder, but as experience is gained, most constructors want their cannon to resemble real ones.
“If you're not a liberal at 20, you have no heart; if you're not a conservative at 40, you have no brain.”--Winston Churchill
Last edited by GGaskill; 04-11-2019 at 02:22 AM.