Are you fund or involved in firearms, maybe like pistols? Then you will or will want to have an idea about the pistol cartridges available. Two of the popular cartridges are .45 ACP or .45 GAP, and beginners usually want to know which cartridge is to be used, especially at the initial training process. The ACP cartridge has been invented since 1904, but it became popularly used as endorsed by the US Army in 1911. This cartridge is cheaper because it is easily available and its larger grip is good for beginners.
The GAP cartridge, which is of a thinner grip implies more expensive ammunition, and it's not easily available. The .45 ACP (that is Automatic colt pistol) was devised by an expert firearms designer, John Browning, and it's designed to use a larger pistol primer. The .45 GAP uses a shorter pistol primer, the cartridge is also smaller when compared to ACP, and they cannot be interchangeably used.