Powder Chamber size for CU's Soda Can Cohorn?

[The Shootist]

 ???I have made a Cohorn from CU's design in 4140, and have seen where some recommend a larger chamber. According to my calculations the chamber should hold about 200 gr. of powder (1"X1.5" chamber size), is this big enough? or do I need to open it up a bit.

Keeping the 1.5" depth of the chamber, how far could I take the diameter (SAFELY)?

I plan to make a "Cannon Lock" and use shotgun primers for ignition. Is the designed 1/8" fuse hole sufficient for this? (I plan to drill a primer pocket in the outside of the fuse hole.

[Cat Whisperer]

The standard rule-of-thumb is to make take the diameter of the powder chamber and ensure that there is at least that thickness of metal all around it - both sides and behind.