Why a shared shell is easier than a shared battleship gun

Could Britain and the United States jointly develop a 16-inch gun in the late 1930s?

It was possible but unlikely, while a common shell programme would be easier to distribute and sustain.

Gun development and testing would have to be concentrated in one industrial base, giving that country an early lead and leverage over its partner. A later change of government could then alter access, priorities, or the terms of sharing. Shell standardization was easier because both countries could produce the ammunition in large quantities through a pooled programme. A common projectile was therefore more practical than a fully shared gun.