2-inch medium mortar

4-inch mortar bomb at the Imperial War Museum

• The 4-inch mortar was the only official type to enter service before the end of 1914 (12 units). It fired an 81⁄2 pound steel bomb with studs to engage rifling grooves in the mortar barrel, similar to the German minenwerfer. The barrel was improvised by boring out a 6-inch shell. Its range of 900 yards and its accuracy was considered acceptable, but the shell case was expensive and was slow to load. About 40 guns and several thousand rounds were issued by June 1915,[8] with 168 of the total 300 manufactured eventually serving in France.[9] It was soon replaced by the 3-inch Stokes mortar in its "light mortar" class.

• The 3.7-inch pipe gun was an early improvised mortar. Eighteen were initially made by the Indian Corps in France by December 1914. It was a smooth tube firing a 41⁄2 pound "tin-pot" filled with ammonal. The fuze was a length of Bickford fuse ignited by the burning of the propellant, which made it too dangerous for long term use. By the end of June 1915, another 100 had been manufactured and sent to France and 20 to Gallipoli.

• The Vickers 1.57-inch trench mortar was first introduced to France in March 1915, 127 by the end of June, and about 275 were manufactured until it was withdrawn in January 1917, after being replaced by the 2-inch mortar. The 1.57-inch was similar to the eventual 2-inch model, but the particular Vickers design was complicated and expensive to manufacture. It fired either an 18 or 33 pound cast-iron bomb filled with Permite (perchlorate), but the smaller bomb was considered too light and the heavier had a range of only 200 yards.[10]

The weapon was initially operated by joint infantry and artillery detachments, eventually it became the responsibility of the Royal Field Artillery.[16] A typical infantry division was equipped with three batteries designated X, Y, Z, each with four mortars.

Its primary use was in cutting barbed wire defenses[17] and attacking enemy front line trenches, such as in the July 1916 attack on the Somme.[14] The spherical shape and relatively low velocity brought the benefit that the bomb did not penetrate the ground before exploding. The short range was a disadvantage as it could only be used if no man's land (between the British and enemy front line) was relatively narrow. It was used to fire some white star (50%-50% chlorine and phosgene) gas bombs during the Battle of the Somme[18] until other specialised longer range projectors became available.

Cordite charges appropriate to the required range were dropped into the barrel before the bomb was loaded. Charges and ranges:[17]

• 1.5 ounces (40 g): 100–220 yards (90–200 m) (dangerous due to propensity for incomplete burn and hence to fall short[17])
• 2.5 ounces (70 g): 180–340 yards (160–310 m)
• 3.5 ounces (100 g): 300–500 yards (270–460 m)

The original design for igniting 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.[3][19] 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.[17] 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.[14]

Estimated rounds required for various targets, with instantaneous percussion fuze 107:[20]

Fuze No. 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

Provision was made in mid-1916 for attachment of the "Temple silencer" at the muzzle,[3] 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.[6]

British Mk V tank disabled by anti-tank mine, Ronssoy October 3, 1918. Unexploded bombs are visible

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 also prevented full clearance of the minefields after the war. This led to some French farmers being blown up in the 1930s when they started using tractors e.g. around Gouzeaucourt.

• Mortier de 58 mm type 2 French equivalent