Measuring my 21st Cent example (and scaling up to 1/16)
wheel diameter 55.7mm, 11.25 wide
Barrel overall 191.8mm (excluding breach assembly - that part is 28mm long)
1st barrel segment 61.8mm long diameter tapering from 10.7 to 9.6
second barrel segment 108 long, 7.7 tapering down to 7 at the muzzle break
Muzzle break 25.7mm long

Hope this helps..