The 5.25 inch Gun was developed in 1942 from the Naval gun. Three, twin mounted guns were handed over to the Army and from these two patterns of single guns were produced. The first, the Mark 1A was purely an Anti-Aircraft gun while the other, the Mark 1B was a dual purpose Anti-Aircraft/Coast Defence gun.
The gun had a loose barel in a short jacket with a horizontal sliding breech block. It had electric firing and air-blast gear. The mountings used a counterbalance weight to balance the muzzle preponderance. The 5.25 inch Gun was completely hydraulic powered. Traversing, elevating, the loading tray and chain rammer, shell supply and fuze setting were all powered. Laying employed remote data transmission to Magslip dials. Open sights were provided for close range aerial targets.
Gun
| Mark 1 | Naval pattern, twin mount only |
| Mark 2 | Land service design. Simpler breech mechanism & heavier breech ring |
| Mark 3 | Land service design using RD Rifling. Trials 1944-46. Cancelled 1948 |
| Mark 4 | Naval design |
| Mark 5 | Land service design. Similar to Mk 2 but automatic air-blast gear |
| Mark 5/1 | As for Mk 5 but slight dimensional changes |
| All subsequent Marks for Naval service | |
Mounting
| Mark 1A | Land service, Anti-Aircraft only. Open back mild steel shield |
| Mark 1B | Land service, dual Anti-Aircraft/Coast Defence. Enclosed armoured turret. Powered shell hoist, fans & louvres for forced ventilation. Semi-auto sights for seaward firing |
| Mark 2A | Naval twin mounting |
Data
5.25 inch QF Gun Mk 2, on Mounting Mk 1A or 1B
| Weight of Gun & Breech Mechanism | 9,587 lbs |
| Total length | 275.5 inches |
| Length of bore | 262.5 inches (50 calibres) |
| Rifling | 36 grooves, uniform Right Hand 1/30 |
| Breech Mechanism | Horizontal sliding block, semi-automatic, percussion fired |
| Elevation | 0º to +70º |
| Traverse | 360º |
| Weight in action | 30 tons 6 cwt – Mk 1A 49 tons 10 cwt – Mk1B |
| Rate of Fire | 8 rounds per minute |
Performance
Firing standard 80 lbs Shell, HE
| Muzzle Velocity | 2,800 feet per second |
| Maximum horizontal range | 27,000 yards |
| Maximum ceiling | 55,600 feet |
| Effective ceiling (with Predictor No.10 & Fuze 208) | 43,000 feet |
Ammunition
| Shell, HE Mark 3C | Square base shell weighing 79 lb 15 oz. Fuze Time 208 |
| Shell, Semi-Armour Piercing Mark 2C | Piercing shell with ballistic cap. Weight 79 lb 15 oz fitted with base percussion Fuze 501 or 502. Seaward firing only |
| Propelling Charge | Seperate loading brass cased charge (full & reduced) |