the powder propellant charge involved the insertion of a standard artillery "T friction tube" into a hole near the base of the barrel. The Royal Artillery had a higher priority in receiving the already insufficient number of tubes so ignition was changed to use a Lee–Enfield bolt mechanism and chamber screwed into a socket in the barrel near the base. A special blank rifle cartridge was loaded and fired via a lanyard from a sheltered position if possible due to the risk of bombs falling short. This ignited the propellant charge and launched the bomb. In early use it was situated in frontline trenches but this tended to attract enemy fire onto the troops manning them. Standard procedure became to locate the mortars separately from frontline trenches, in unoccupied trenches or in saps running off the frontline. This had the benefit of drawing enemy fire away from troops manning the front line. Estimated rounds required for various targets, with instantaneous percussion fuze 107: Cutting barbed wire: 1 round per 10 square yards (8 m2) Cutting loose wire: 1 round per 6 square yards (5 m2) Destroy trenches frontally: 5 rounds per yard Destroy trenches in enfilade: 2.5 rounds per yard Destroy a machine-gun emplacement with top cover not more than 3 feet of earth: 80 rounds Silencer Provision was made in mid-1916 for attachment of the "Temple silencer" at the muzzle, intended to reduce the flash and noise generated on firing, which at the short ranges the mortar operated at was quickly noticed by the enemy and invited artillery response. This required the use of bombs with a special piston attached to the tail which was retained in the barrel by the silencer on firing, and hence sealed the muzzle after the bomb tail left the barrel. This had the major disadvantage of causing the barrel to overheat during prolonged daylight firing, and the silencer was only used at night. Use as anti-tank mines In Spring 1918 many of these obsolescent bombs were buried on the Western Front under metal plates as anti-tank mines in expectation of attack by German tanks. This led to some later confusion as to whether unearthed bombs were unexploded mortar projectiles ("duds") or undetonated mines. These minefields were inadequately documented. This caused the British problems in the closing months of the war when they had to advance again over territory they had previously abandoned and