The 18 pdr Gun was introduced in 1904. More than 10,000 were made of the original model. Work began in 1916 on an improved design but it wasn’t until 1918 that it went into production as the Mark 4 Gun on Mark 3 Carriage. This was a big improvement with a box trail and Asbury breech mechanism. It became the standard field gun in the post-war years. It was improved still further in later years with a split trail carriage and pneumatic tyres. By the end of its life there had been five Marks of Gun and five Marks of Carriage with many sub-Marks in each.
When the 25 pdr Mark 1 was introduced, a number of 18 pdrs were coverted by simply changing the barrel. As the 25 pdr took some time to come to full production, the 18 pdr stayed in service for most of WW2. Eventually, the Mark 4 was made obsolete in August 1940 as the last one was converted to 25 pdr. The rest remained in use until 1944 having been used for training and then anti-invasion defences in 1940-41. They were also used in the Middle and Far East.
Gun
| Mark 1 | Original pattern. Wire-wound, screw breech, 1904 |
| Mark 1* | Mk 1 repaired by insertion of new ‘A’ Tube and wire. Gun assembled by hydraulic pressure |
| Mark 2 | Gun assembled by hydraulic pressure instead of heating/shrinking 1906 |
| Mark 2* | Mk 2 converted for AA mounting in 1915. Converted back in 1920 |
| Mark 3 | Development model not adopted 1917 |
| Mark 4 | New design, Asbury breech mechanism 1918 |
| Mark 4A | Mk 4 repaired by replacement of ‘A’ Tube & wire by auto-frettaged loose liner 1935 |
| Mark 4B | As Mk 4A but new manufacture 1935 |
| Mark 5 | As Mk 4 but differences to suit Birch Gun mounting 1926 |
Carriage
| Mark 1 | Original, pole trail, hydro-spring recoil, elevation 16º 1904 |
| Mark 1* | Mk 1 modified to hydropneumatic recoil system 1916 |
| Mark 1R | Mk 1 with solid rubber tyres |
| Mark 1*R | Mk 1* with solid rubber tyres |
| Mark 1** | Mk 1 * with lengthened cradle |
| Mark 1C | Mk 1 with extended cradle |
| Mark 1D | Mk 1 with Mk 2 Carriage cradle |
| Mark 2 | Similar to Mk 1 but hydropneumatic recoil system & longer cradle |
| Mark 2* | Mk 1* but with Mk 2 Carriage cradle |
| Mark 2PA | ‘Martin Parry’ conversion, pneumatic tyres on steel disc wheels |
| Mark 2R | Mk 2 with solid rubber tyres |
| Mark 3 | For Mk 4 Gun, box trail, extended cradle, hydropneumatic recoil, elevation 30º |
| Mark 3T | Mk 3 modified to approximate Mk 4 standard |
| Mark 3* | As for Mk 3 but elevation 37½º |
| Mark 4 | As Mk 3 but new trail & traversing gear. Shock absorbers between axle & trail, elevation 37½º |
| Mark 4R | Mk 4 with solid rubber tyres & elevation 38º 40′ |
| Mark 4P | Mk 4 with pneumatic tyres & elevation 37½º |
| Mark 4TR | Mk 4 but with only 30º elevation |
| Mark 5 | Split trail, 50º traverse, 38º 55′ elevation |
| Mark 5R | Mk 5 but with solid rubber tyres |
| Mark 5P | Mk 5 but with pneumatic tyres, 37½º elevation |
Data
Gun Mark 4 on Carriage Mark 5P
| Weight of gun & breech mechanism | 952 lbs |
| Total length | 96.96 inches |
| Length of Bore | 92.735 inches (28 calibres) |
| Rifling | 18 grooves, uniform Right Hand 1/30 |
| Breech mechanism | Asbury interrupted screw, precussion fired |
| Elevation | -5º to 37½º |
| Traverse | 25º right & left |
| Recoil system | Hydropneumatic, variable, 26 to 48 inches |
| Weight in action | 3,507 lbs |
Performance
Firing standard 18 lb HE Shell
| Muzzle velocity | 1,625 feet per second |
| Maximum range | 11,100 yards |
Ammunition
Fixed case charges
A huge number of shells & cartridges were approved during the gun’s 40 years service. Listed below is a representative round of each type.
| Cartridge, QF, 18 pdr HE Shell Mark 46 | Non-streamlined shell filled with Amatol or TNT. Percussion fuze No. 106 or 106E. 11.6 inch brass cartridge case with 1 lb 7 oz of Cordite |
| Cartridge, QF, 18 pdr HE Shell S/L Mark 13C | Similar to previous with streamlined shell. Fuze Percussion No. 115, 117 or 119 |
| Cartridge, QF, 18 pdr HE Shell Reduced Mark 14 | Similar to Mk 46 but with propelling charge of 5¼ oz of Cordite |
| Cartridge, QF, 18 pdr Smoke Bursting S/L Mark 11C | Nose fuzed streamlined shell filled with White Phosphorus & Fuze Percussion No. 117 |
| Cartridge, QF, 18 pdr Shrapnel Mark 63 | Nose ejecting shrapnel shell Mk 17 containing 375 bullets. Time & Percussion Fuze No. 80 |
| Cartridge, QF, 18 pdr AP Shot Mark 3T | Solid steel piercing shot with tracer |