@EternalDarkness @AstleyIndustries Thanks for typing in the exact size (0.304)
An easy way to make a cannon very accurately sized is to make a 0.304 (for 152mm) sized tube, put it in the middle of the gun barrel (intake) and scale the inlet thickness to perfectly match the 0.304 block.

I have no idea

@MechWARRIOR57 I don't know. Place a 2x2x2 block, nudge the cockpit inside it, and upload. It gives metric units on the site, which will show you it's 1x1x1meter

@EternalDarkness then why is it that in the properties section, it uses ft?

@MechWARRIOR57 blocks are metric. One block is 0.5m (50cm). 152mm is 0.304 SP units.

a way to see this is to use fine tuner to make the cockpit really thin, then use a calculator to see what 152mm is in feet, which is 0.4 feet, to make a fuselage that is 0.4 feet

1 block corresponds to 50cm (500mm), and 1m corresponds to 2 blocks. So, 152mm is 0.304 blocks. Remember to factor in barrel thickness as well. So, you may want a fuselage inlet 0.35+ in diameter.
I used a 0.4 block diameter for an inlet to achieve a 155mm caliber. I had to approximate the inlet thickness so it looks realistic. But then, it was my first legit cannon

@Himynameiswalrus