[Mickey Rat]

45 ACP (automatic Colt pistol) is a designation for all John Browning designed pistol cartridges:

25acp
32acp
380acp (also 9mm kurtz)
45 acp

All were new cartridges designed to work in the newly invented semiautomatic pistols in the very early 1900's.

45 Colt (often called Long Colt) is the cartridge used by the 1873 Colt Peacemaker, a black powder round.

Many of the older black powder cartridges are rather large compaired to more modern cartridges. This is because black powder has much greater volume than smokeless (modern) powders. A 45 Colt cartridge is almost full of black powder. A full case of most smokeless powders would cause the gun to explode.

Guns and Ammo recently ran an article compairing the 44-40 (a black powder round) and the 40 Smith & Wesson (a modern smokeless powder round). Although over a 100 years difference in their introduction, they are balistically identical.

I personally love the 45acp. I have at least 7 pistols in that caliber. I have a Ruger New Model revolver and a Stainless Steel Copy of the 1892 Winchester lever action rifle. I enjoy shooting both.

Either the 40 S&W or the 45acp are wonderfull self defense rounds. Pick the gun you like best, and buy it. Then shoot it untill you become proficient with it.