Mk III and IV carriages incorporated a new variable-recoil hydro-pneumatic system and moved from above to below the gun barrel. The new single-motion "Asbury breech" allowed for higher rates of fire and a Welin tapered screw adopted for the breech. A single battery of the Mk IV gun on the early Mk III carriage was serving with 4th Army when the First World War ended. The new gun and carriage was in effect a new weapon but as the calibre and ammunition remained the same it was referred to as part of the 18 pr development cycle until the calibre was phased out.