QF Single 4" Gun

The Quick Firing (QF) 4-inch was developed in WWI and used through WWII and beyond. It was mostly used as a high-angle anti-aircraft gun, though it was described by John Campbell as having "neither long barrel life nor particularly high accuracy."
Nonetheless, this gun was employed on a variety of Commonwealth ships and was a popular weapon during WWII, to the point that production struggled to keep up with demand and many ships had to settle for older models. 
The QF 4-inch gun Mks I, II, III were early British quick firing naval guns, all with barrels of 40 calibres in length. The gun was intended for Naval service replacing the QF 3” and QF 12-pounder. 
This gun is a QF 4-inch XVI from the Flower-class corvette HMCS Calgary, whose gunshield graffiti emblem is painted on the side.

Reference Links:
Ministry for Heritage – 4-Inch QF Gun, Hayne's Cave Battery
For Posterity's Sake – Gunshield and Shipboard Art on ships of the RCN currently in commission
Naval Museum of Alberta – HMCS Calgary
NavWeaps – United Kingdom/Britain 4”/45 (10.2 cm) QF Mark XVI and Mark XVI*