The 45 ACP (Automatic Colt Pistol) IS a straight-walled pistol cartridge. Matter-of-fact it is a "rim-less" cartridge, the only one I can think of off the top of my head.

A straight-walled pistol cartridge refers to the fact that there is no necking (down) to a smaller caliber bullet, such as most rifle cartridges. The intent of the rule is to rule out the use of single-shot pistols which can shoot rifle cartridges, such as the TC Contender chambered in a .308.

So by definition (ethics regardless) anything with a straight-walled cartridge, larger in diameter than or equal in diameter to .357 inches is perfectly legal. This makes something like a 9mm, .40 S&W, and a .45 ACP legal.