What's the diameter of the shell base in millimeters?? Because its almost has wide as the projectile itself (just a bit wider), it should give a good indication of the size of the cannon.

For the U.S at least, I think the largest one piece round was a 5 inch naval gun (slightly larger than a 120 mm cannon). The 155 mm cannon and 8 inch howitzer had separate projectiles and powder charges. Big naval cannons as well, although I don't know about heavy cruiser naval cannons (8 inches).