The .45 ACP shot shell was developed for the purposes of a survival round for servicemen, primarily pilots. There were 2 different versions and 3 different names.
The first was the T-23/M12 which was developed in late 1943. It was actually an adaptation of the Remington-Peters shot cartridge which had been used with the Thompson SMG prior to the war. It had a long paper bullet filled with 103 pellets of number 7 1/2 shot and was slightly improved to carry 130-140 pellets in the military T-23 version.
An R&D contract was given to Remington for 10,000 rounds of T-23 shot shells with the markings "Use only for huting game. Do not use against enemy troops". This was also written in German and Japanese.
The M12 was never fully satisfactory in the field. The paper shot casing absorbed moisture which led to misfires and swollen rounds that would not chamber. In addition, the M12 paper projectile would often fail to open properly causing the pellets to group poorly. The M12 had to be loaded individually because of its excessive length. These problems led to the adoption of the M15 round with its superior wad and waterproofing.
The M15 was adopted Dec 1943. It had approximately 118 7 1/2 chilled lead shot pellets. It functions through a standard magazine and fully operates the action.
Supposidly in 1945 the Army Air Force included a smooth choke-bore barrel in the 'jungle kit' to improve the shot pattern. However records indicate that this was never actually issued.
It is not recommend to shoot these in modern weapons as they all of these had corrosive primers. And they are more valuable to collectors.
Today reloaders can load their own shot shells utilizing RCBS dies or home made dies.