0.60

The .60, sometimes referred to (inaccurately) as the T17, is a rimless, bottlenecked rifle cartridge developed in the United States beginning in 1939. It was never used in service, but was enlarged to become the basis for the 20x102, which remains the standard US aircraft round to this day.

History
The .60 was originally intended to be an anti-tank rifle cartridge capable of penetrating 1.25 in of armor at 500 yd, a step up from the 0.9 in at 200 yd of the 0.50 Browning. By the time it was ready for service, tanks invariably had much more armor than the 0.60 was designed to penetrate, and it was not put into service in its original role.

The Germans had developed a very similar 15 mm round, and like the US, abandoned it when it proved too small for use against newer tanks. They then developed a new heavy machine gun based on it, the MG 151/15, primarily as an aircraft weapon. A number of these were captured early in the war, and some made their way to the US. As the German 15 mm round was almost identical in performance to the US 0.60, the MG 151 was quickly adapted in a similar weapon known as the T17 Heavy Machine Gun. This did not reach service during the war.

In the immediate post-war there was some development of a system mounting four such guns and a radar in a towable mounting known as the "Stinger", proposed as a new anti-aircraft gun to replace the M45 Quadmount. Development continued until 1951, when a change to require longer range led to a series of events that would lead to the FIM-43 Redeye a decade later.

The US Air Force (USAF) also considered the T17 as an aircraft gun, as well as a version using the same cartridge but necked down to mount the .50 BMG bullet known as the .50/60. The .50/60 had a very high muzzle velocity, over 4000 foot/s, which made it much easier to aim in combat. The resulting weapon lacked power, as did the original 0.60, and neither went into service.

During the war, the Germans had concluded the 15 mm round lacked power in the aircraft role, and had developed a new 20 mm round by necking out the cartridge and fitting it with the "mine shell" from the MG FF cannon. This formed the basis of the MG 151/20, which would go on to be their primary aircraft weapon for the rest of the war. The USAF, reaching the same conclusion with their 0.60 experiments, did the same to adapt the 0.60 to a 20 mm explosive shell as part of their M39 cannon project. This became the 20x102, which remains the standard US 20 mm round to this day.