You see, the whole reason .40 was developed was to give law enforcement a more powerful pistol than 9mm. Yes, the .45 ACP was out, but .45 pistols were also very limited by capacity.

For decades the FBI claimed the .40 Smith & Wesson was the optimal personal defense round. But the 9mm versus .40S&W battle rages on. (Photo by FBI) Defensive caliber arguments have been going on ...

Today there are more choices of .45 ACP pistols than ever before, as almost all gun manufacturers offer their most modern semi automatic handguns in the big caliber.

Both the 10mm Auto and 45 ACP were born out of a desire for a more powerful pistol cartridge. Here's what to know about the 10mm vs 45 ACP.