I have been loading some .45acp and my die kit came with a crimping die. What is the purpose of this die? Is this step optional or should I be crimping my cases?

It should have a crimp. You normally bell the mouth when you are loading an ACP alight crimp will restore the case.
Also many pistol powders work best with a good firm crimp allows them to build consistant pressure.

Crimp only if the bullet has a cannelure. Crimping a bullet without one can cause the jacket to ripple on the core and bores don't like that so much. My gunsmith told me that. What would he know? He was only in it for 45 years and built "special purpose" rifles for Uncle Sam. Care to guess the purpose of those?

Yes, handgun rounds should be crimped. I've pretty much gone to a taper crimp rather than a roll crimp.

quote:

Originally posted by homebrewer:
Crimp only if the bullet has a cannelure. Crimping a bullet without one can cause the jacket to ripple on the core and bores don't like that so much.

For rifle, and revolver (both normaly roll crimped) that's good advice, but bullets for self loading pistols are normaly NOT canalured and the application of a taper crimp (what's normaly used with auto pistol ammo) dosn't require one.

Apply a slight taper crimp, IE the case at the mouth should be .004-.005 smaller than it measures near the base of the bullet.

quote:

Originally posted by Dead Eye Dick:
I have been loading some .45acp and my die kit came with a crimping die. ...

The 45ACP Headspaces on the Case Mouth. So, you want to use either a Taper Crimp(most likely what you have) or a Profile Crimp.

You do not want to use a Roll Crimp, or any Crimp that would turn the Case Mouth "into" the Bullet. I doubt any Die manufacturer is making a Roll Crimp for any Cartridge that Headspaces off the Case Mouth today, unless it would be a special order.

So, you do want to use the Crimp Die, just use it enough to push the Flair back up against the Bullet. Then take the barrel out of your Pistol and drop a Cartridge into the Chamber. Look across the Case Head and you can tell if it is Seating Properly. If you are not familiar what it looks like, use a Factory Round to get the proper perspective.

Trimming is fairly important since they do Headspace on the Case Mouth.

Best of luck to you.

Your Lee die set comes with a seating that that will also apply a "taper" crimp. Crimp only enough to remove the flare.
Your set also came with a Factory Crimp Die. It applies the same taper crimp but in a separate step from the seating process, but it also has a carbide ring at the base. This carbide ring works similar to the sizing die and smoothes the finished round out to just the right specs,diameter for flawless function in your semi-automatic pistols.
Try it you will love it.

Taper crimp 45 acp .002" smaller than the case body. This will be around .471" +/- .002" Sounds like you might have Lee dies with a FCD. "Seating die crimp style

There are two crimp shoulders in our bullet seating dies. The first shoulder applies a slight taper crimp and the second shoulder applies a full roll crimp. The closer the die is adjusted to the shell holder the heavier the crimp will be." Lee Tech Assistant