OTHER GUNSIGHTS USED BY THE USA(A)F
Besides its own Gunsights, the USA(A)F used a few other sights supplied either by the US Navy or the R.A.F. during WWII and the early post war years. As usual, this section will only cover gunsights developped until the early fifties and not any “modern” sight. It will also not cover gunsights fitted to USAAF planes produced under licence elsewhere (ex: the CAC Mustang) or planes purchased from other services or countries (ex: Navy Skyraiders).
The L-3 (or LY-3N), designed and developed by the Lynn Instrument Company, had the distinction of being the only sight used but not directly procured by the Air Force. It was tested and approved by the AAF Proving Ground in December 43.
Its operational use was limited to the following types:
- P-61 all types
- P-38 from P-38J onwards as the redesign of the cockpit windscreen prevented the use of the N-3 previously fitted. The L-3 was however considered only as a stop-gap measure until the N-10 gunsight would become available (but this never happened).
- P-51D. Although no documentation has (yet) been found, it is strongly believed that North American installed L-3 sights on some early P-51D as production, at times, exceeded gunsight availability. Moreover why would pilots and/or fighter squadrons, with no previous operational experience with the L-3 gunsight, field modify P-51D equipped with N-9 back to L-3 which had lower optical and eye aperture performance?
- P-47. A few L-3 were send in March 44 to the 56FG for operational testing and at least one of them, fitted to P-47D “Lady Jane”, scored a victory.
Another company, the Na Mac Products Corp., later converted L-3s for rocket firing (by installing a different reticle) for installation in P-38, and also started to sell refurbished(?) L-3 to Lockheed. This is rather strange as the L-3 gunsight was patented by Lynn in Feb 1944.
Lynn also developed and proposed the L-9, basically an L-3 with a variable head calibrated for 15 degree angle adjustment. But it was rejected by the AF after testing late 1943 and therefore never produced.
British Mark II (and Mark IIL) Gunsight
As the British Mark II Gunsight was considered better (bigger 3,5 inches aperture and better optical performance) than the standard the N-3A or B gunsights being supplied at the time, most P-47 and P-51B&C arriving in Europe and as many as possible P-40 were modified at Air Depots with the British Gunsight before being delivered to active units. Note that this situation later led to the order of Mark 8 Navy gunsights as a stop gap measure for the P-47 (see chapter for the Mark 8 below). Later on, some 9th Air Force P-47 were also fitted with the Mark IIL Gunsight; basically a Mark II with a variable sight head used for low level bombing and rocket firing.
Some 8th Air Force P-51D were also modified in Europe to use the Mark II. This is quite puzzling as P-51Ds were already fitted with N-9 gunsights at the factory and the N-9 was unanimously recognized as far better than the much older British Mark II. Maybe those Mustangs were some of the few P-51D that could have been fitted at the factory with the Lynn L-3 gunsight (see chapter on the L-3 gunsight above).
US Navy Mark 8 Gunsight
As most P-47 reaching the European theater were modified to use the British Mark II gunsight (see above) before being sent to combat units, Material Command requested in July 1943 that, as an interim measure and until the new N-9 gunsight would be available in sufficient number, all P-47 leaving the production line should be equipped with the Navy Mark 8 gunsight. An order of 5.400 sights was therefore placed in October 1943 (later increased to an unkown number) and the Mark 8, starting with the P-47 D-20 series, finally remained the standard P-47 gunsight until gyro K-14A gunsights became available in sufficient number early 1945.
Period photos also show that at least a few P-51D in the Pacific theater were field modified to use the Mark 8 gunsight. Strange as the Mark 8 was definitely not superior to the N-9 that was the standard equiment for early P-51Ds. So why replace the N-9 with an inferior equipment? Maybe a few ex P-47 pilots that were nostalgic of the Mark 8 that they used in their previous fighter.
It’s also interesting to note that the Mark 8 also saw post war service with the Air force as the Mark 8 Mod 8 fitted with a Mark 4 Adjustable Reflector was part of the interim fire Control system fitted on early Martin B-57 Canberra. The Army also used it at least on some of its Huey UH-1 gunships.
US Navy Mark 20 Mod 4 Gunsight
The Navy Mark 20 Mod 4 gunsight is a “specialized” gunsight, developped by the Navy from the well proven Mark 8, fitted with 3 types of reticle patterns and light filters (for respectively night, dusk and day use) to increase the operational scope of nightfighters and some attack aircrafts. It was ordered and used by the Air Force on the B-26K Counter Invader, some early versons of its A-37 Dragonfly and some other special role aircrafts like the AC-47 and the T-28D (?).
US Navy Mark 18 & 21 Gunsights
As the Navy interest in the adaptation and production of the British Mk II GGS lead computing gyro gunsight happened much earlier than for the Air Force (at least that’s what the Navy said), production of the USN Mark 18 gyro gunsight for flexible gunnery was already under way late 1943 when the Air Force only started to show strong interest in such a sight.
As the Air Force own production facilities would not be able to start mass production of their version of the British sight (the K-14A) before end 1944, they immediately put a lot of pressure on the Navy to:
- Develop and produce quickly a fixed gunnery version of the Mark 18 gunsight to be used on fighter airplanes. This gunsight was designated the Mark 21 by the USN and K-14 by the Air Force; and
- Allocate urgently as much as possible of those gunsights to the 8th Air Force in Europe.
The Air Force apparently succeeded as most, if not all, of the Mark 21 production was finally allocated to the Air Force. In fact, I have never found any reference of the Mark 21 being actually used on any late or early post war Navy fighter plane.
For the Mark 18, contrary to what has often been written, it has never, besides a few prototype and experimental installations, been used on Army Air force fighters. However, after WWII at least North American and Republic Aviation modified a batch of Mk-18 sights for use in some of their early production F-86A and F-84. This was most probably due to the acute shortage of available K-14 prevailing during that period.
 U.S. Navy Bureau of Ordnance in WWII – Department of the Navy – page 347
 Handbook of instructions for K-14A Gun Sight – AN 11-35C-1 dated April 1945