OP 909 20MM AA GUN
OP 911 20MM AA GUN
OP 1439 20MM A. A. ASSEMBLIES (TWIN)
Refer to OP 911 CHAPTER 9 For the ammunition for Guns Marks 2 and 4
Interchangeability
There is very little difference, from the operating point of view, between
Mark 2 british and marks 2 and 4 u.s.n.
All of the above three marks of assembled guns are interchangeabile units. There are certain differences in the marks 2 and 4 guns that affect interchangeability of parts. When individual parts are not interchangeable between marks 2 and 4 the assembly or the subassembly containing these parts is usually interchangeable.
In the first year of the war the 20mm gun and the 1.1" gun shot down the most aircraft of any gun system in the fleet.
THE MARK 14 Gun Sight was first adapted to the 20mm and 1.1" gun in about the mid point of 1942 as it became available. These were the primary air defense weapons systems at the time. The 20mm served by the thousands until the end of the war and beyond.