Well they vary they start from 1" and go to Beer Can . My first and one of my favorites is a custom casting from an aircraft contractor., They did it with 80% stainless and 20% Nickel it is 30" long X 1.75 bore in a Queen Anne configuration The trunnion on it is well below center .., its special to me. Its from 1990.
Also I have a 1" made from 3" brass stock by Brooks USA its 12" in length..., Its a BIG lilttle gun .
I guess ya gotta go with what you can get? Good steel 1018 is tough.
Brass and Bronze are expensive today.,its like buying money.
I am not a machinist., but I would start with steel and save., then with experience and some money saved .,work for that cannon or mortar design you really want the most......, in a scaled down version..., and then do it in a bronze.
I am very proud of my Beer can mortar., its alot less material as in only 12" long including trunnion but with a beer can bore.
I think its the most for the least. I would say for one gun., its my favorite to shoot. A bigger boom and a shorter range with a visible projectile..., Mortars are just plain fun to watch !
In a cannon., the most for the least in my humble opinion is the 1" bore.
I have 3 in Cast Iron and a Steel tube liner. I love them. They are my biggest guns.
However my other 8 cannons are of stainless (Most don't like it.., but it doesn't rust on the boat) Brass and Bronze and they are FAR more maintenance free and much less on my mind about greasing and maintenance.
I am just simply not funded for HUGE BIG SCARY BRONZE GUNS or I would have gotten them as oppose to the steel.