the gun's range to 6525 yards in normal use. The range could be increased to 7800 yards by "digging in" the end of the pole trail to increase elevation. Its distinguishing feature was the barrel, significantly longer than the 13-pounder's and, unlike the 13-pounder, the barrel was significantly longer than the recuperator housing above it. Mk II Gun on Mk I Carriage The original gun design was quickly replaced in production from 1906 by the "rationalized" Mark II gun for ease of relining : the exterior of the inner "A" tube was slightly tapered, and was inserted into a matching tapered jacket by hydraulic pressure. Mark I and II guns were still in use in the post-First World War era, and some saw combat in the Far East in Second World War. Mk II Gun on High-angle Anti-aircraft mounting In early 1915, a number of 18-pounder guns were mounted on pedestals, with the addition of a second recuperator and retaining catch for the cartridge case at high angle, in an attempt to come up with a workable anti-aircraft gun. The gun's relatively low muzzle velocity, and the unsatisfactory ballistic characteristics of its shrapnel shell at high angles, made it a marginal performer in such a role. However, by sleeving the barrel down to and mating the 18-pounder cartridge with the 13-pounder's shell, the successful QF 13-pounder 9 cwt anti-aircraft gun was produced with the necessary high muzzle velocity. The early versions of 18-pounder anti-aircraft guns remained in service, apparently only in the home defense of Britain. 35 were in service in Britain in June 1916 and 56 at the end of the First World War. After the war, they were converted back to field gun use by removing the cartridge retaining catch. Mk II Gun on Mk I Carriage with armoured oil reservoir on recuperator Battlefield experience in 1914 and 1915 showed up the weakness of the original recuperator springs (which returned the barrel to firing position after recoil) and loss of oil in the recuperator under intense firing. Poor quality of wartime manufacture of the springs was also a factor. Spring shortages due to breakages meant guns remained in the firing line and had to be "run up" – have the barrel moved forward into its firing position – by hand, hence slowing the rate of fire. A temporary preventive measure was the addition of a distinctive armoured box-shaped