Caliber Length
Caliber length (barrel length), in the artillery, refers to the length of a gun or howitzer barrel from the muzzle to the base of the breech, in relation to the caliber. The barrel of the 7.7 cm L/23 Krupp M.96 n.A. field gun, with 23 caliber lengths and a caliber of 77 mm, is therefore 23 × 77 mm = 1771 mm long.
The barrel of the 7.5 cm L.46 PaK 40 (= 3450 mm), pictured above, should be 39.7 mm long in 1:87 scale, but we are actually measuring 43.5 mm here.