.45 ammunition questions

Question
Hi Dave, I have a couple of questions regarding my Ruger P-90 .45 ACP. I've had the gun for a few years now and have always used .45 ACP ammunition.  I know that ACP stands for Automatic Colt Pistol, but I don't know if this is the only kind of .45 ammunition I can shoot.  This probably sounds stupid, but I don't even know if ammunition marked ".45 Automatic" is the same as .45 ACP?  Could you please clarify for me?  I'm really just trying to make sure I always have the right ammunition for this gun.  A dealer once told me to only buy the .45 ACP ammo, is this right?
Thanks for taking the time,
Sincerely,
Ed Rosek
Las Vegas, NV

Answer
If it says .45 ACP or .45 automatic, or .45 auto, or .45 auto-loading on an ammo box, any one of those is OK. If any ammo is called .45, it is going to be the same as .45 ACP with two exceptions:

1.  .45 Colt (also called .45 Long Colt or .45 LC)

2.  .45 Schofield (also called .45 Smith & Wesson or .45 S&W)

These others are cartridges made for revolvers, and are easily distinguishable from the .45 ACP.  .45 Colt is used in old style guns for cowboy reenactments, etc. You'll probably never see a .45 Schofield.

The rim at the back of a .45 ACP has a diameter about the same as the diameter of the case. But on revolver cartridges, the rim diameter is larger to keep the cartridge from falling into the chamber of the gun's cylinder. The revolver cartridges also have a longer case than the .45 ACP.

Maybe you can find a photo online comparing the .45 ACP and the .45 Colt (or Long Colt).

Hope this helps!

Dave