The 12 pdr Coast Defence Gun originated in 1894 as a naval gun for defence against fast, light craft. It remained in service, with minor modifications, until 1956. One aspect of the original cartridge was odd in that it had a protruding electric primer. It could be replaced by a similar adapter to take an electric or precussion tube. This resulted in the breech block being recessed to fit around this primer block. After firing, the case was extracted by a gun number using a forked hook on the rim of the primer to pull the cartridge case from the breech. This sytem was replaced in 1935 by conventional extractors engaging the cartridge rim, although some older guns remained in service.
The mounting was a simple pedestal and a shield. Early guns had gear driven elevation and traverse. Later this was changed to a free swinging traverse. The layer hooked his arm over an arm piece and swung the gun round. Virtually obsolescent by 1939, a large number were brought from store and emplaced around the coast in 1940. These were additional to those already in place in fixed defences.
Gun
| Mark-1* | Converted to Anti-Aircraft use in WW1. Re-converted to coast use later |
| Mark-1** | Mark-1* converted to Mark-4 standard with automatic extraction |
| Mark-1*** | Mark-1* converted to Mark-4 standard |
| Mark-4 | New pattern extractor & breech carrier to allow automatic extraction. Introduced in 1935 |
| Marks-2, 3 & 5 | Naval service |
Mounting
| Mark-1 | Shield & pedestal with 18 holding down bolts |
| Mark-2 | As Mark-1 but with shorter elevating arc, simpler shield & pedestal design. Weight in action 6,328 lbs |
| Mark-3 | As Mark-2 but lighter and minus the geared traverse. Weight in action 5,432 lbs |
Data
| Weight of gun & breech mechanism | 1,395 lbs |
| Total length | 123.6 inches |
| Length of bore | 120 inches (40 calibres) |
| Rifling (guns were brought up to new standards when going for periodic re-furbishing) | Originally increasing, right hand, 1/120 at breech – 1/28 at muzzle Then straight grooves to 83.035 inches from muzzle & increasing to 1/30 at muzzle Finally uniform, right hand, 1/30 |
| Breech mechanism | Interrupted screw, single motion, electric & percussion fired |
| Recoil system | Hydro-spring 12 inches |
| Elevation | -15° to +20° |
| Traverse | 360° |
| Weight in action | 9,240 lbs |
| Rate of fire | 15 rounds per minute |
Performance
Firing standard 12 lb 8 oz HE shell
| Muzzle velocity | 2,257 feet/second |
| Maximum range | 10,100 yards |
Ammunition
| Shell QF 12 pdr HE Mark-5 | Forged steel shell with Fuze Percussion DA No.44 or 45 |
| Shell QF 12 pdr CP Mark-5 | Pointed shell with Fuze Base No.12F Special for penetration of light structures |
| Shot QF 12 pdr Practice Mark-3 | Plain steel shot simulating the common pointed shell for practice |
| Cartridge QF 12 pdr 12 cwt Mark-3 | Brass case with igniter at the base. An adapter was screwed into the base taking a Tube Vent Electric. In the event of failure of the firing circuit a Tube Percussion could be fitted instead |