gun, Mk II was a British light artillery piece designed during World War I.

[1] Its light construction dictated a low muzzle velocity, which resulted in it having a short range.

It was too light to withstand the detonation of standard British explosive propellants, so its ammunition used ballistite packed in cambric bags instead.

[2] With the gun no longer wanted in the trenches, it was decided to adapt it for use on aircraft as an air-to-air weapon against airships and observation balloons and for use against ground targets such as trains, ammunition dumps, and tanks.

100 Squadron, however, reported problems with the gun: Its report stated, "Sometimes the shell does not leave the barrel for some time after the striker has been released;"[2] in one incident, a gunner who thought the gun had misfired was about to open the breech to remove what he thought was a hung round when the shell went off in a shower of sparks.

The Royal Aircraft Factory N.E.1 night fighter was also constructed to the same specification to carry the gun; though it flew well, it lacked the performance for use as a night-fighter.

[2] After the failure of both the F.B.25 and N.E.1 to win production orders, interest in operational employment of the 1.59-inch breech-loading Vickers Q.F.