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.