A couple ?'s for y'all. First of all can someone explain what the case trimming does? i am new to this and just reloading pistol rounds for now and they are just practice rounds so I'm not concerned with accuracy. The steps I am now taking are cleaning, resizing the casing and placing new primer, flaring the neck and adding powder, then finally seating the bullet and then I check to see if the overall length is right by means of a caliper. Am I missing any steps?
Second ? is when reloading .45 colt, the lead bullets have a grove around the circumference that I believe is where they are supposed to meet the casing crimp. When I place crimp on the groove the OAL is 1.580-1.585. The data I have says they are supposed to be 1.600. So about .015-.020 shorter than what it is supposed to be. Is this difference significant??
Also, these were new brass so I don't know if that matters or not.
Thanks

If you are loading for semi-auto's, the cartridge headspaces on the case mouth and length is critical.

For revolvers, trimming isn't required that often, other than keeping the mouths square and being able to crimp at the right location.

The 1.600" for the 45LC is the max length. Your OAL's of 1.580" - 1.585" is really nothing to get worried about. Better to be short than too long.

You're doing OK with your reloading. Only step you might consider adding is to use the caliper to measure case length AFTER FL resizing to assure the cases haven't gone beyond maximum. When flaring (or belling) the mouths, try to flare the minimum amount to get the bullet started. If loading cast or plated bullets, the use of a Lyman "M" die will aid considerably and you don't have to flare the mouth in advance.

Except to uniform case lengths, or to turn a .357 case with splits into a .38 Special without splits, I've never seen anybody trim pistol brass.

Most pistol cartridge cases have a problem with shortening in use. Rifle cases lengthen a lot the first firing, then slowly thereafter. Trimming rifle brass is quite time consuming, and is tedious when you are doing a large batch.

CDD

"If you are loading for semi-auto's, the cartridge headspaces on the case mouth and length is critical."
That is true for straight-walled rimless cases like the 45ACP and 9mm Parabellum. For semis, like the S&W 52, that use rimmed cases like the .38 Special, the cartridge headspaces on the rim. For the few semis that use bottlenecked cases like the .400 Corbon or the 5.7X28 or the .357 Sig, the headspace is on the shoulder.
Pete

My .45 ACP cases shorten, as will any brass that isn't at pressure high enough to stick the case to the chamber wall against the rearward thrust of reacting to the bullet moving forward. I can still get 50 reloads or so from them making light target loads by headspacing on the bullet instead of on the case rim. It is my observation that many auto chambers are short enough that the rounds actually headspace on the extractor hook before the case mouth makes it to the end of the chamber. That's not conducive to best accuracy.

In revolvers, trimming is most often just to uniform the brass so the same crimp die setting produces the same strength of crimp each time. That can affect both accuracy and burning uniformity for some powders. Some of the higher pressure revolver rounds stick the brass and grow it. These need trimming so the case mouth doesn't jam into the chamber throat and raise pressure because of being unable to open up and let go of the bullet. That is the reason rifle cases must be trimmed, unless they are carbines shooting low pressure pistol rounds.

Unclenick-- not sure what you are talking about when you say headspacing on the bullet ?

Unclenick-- not sure what you are talking about when you say headspacing on the bullet ?

Headspace is the room the cartridge has in the chamber. Headspacing is just determining where the cartridge will stop going further forward when it enters the chamber or is knocked forward by firing pin impact, because where it stops determines how much extra space is left behind it. The headspace determinates are the parts of the cartridge and its counterpart in the chamber that do the stopping. The .45 ACP, like other straight wall rimless self-loader cartridges, was designed to use the case mouth as the headspace determinate. On chambering, the case mouth stops the round going further forward when it bumps into the step in the chamber where the case mouth profile is cut and where the chamber narrows down to the entry into the throat.

If your cases are too short, the extractor hook will stop the cartridge from going further forward before the case mouth meets the chamber step. At that point the extraction groove in the case and the extractor hook become the headspace determinates and the cartridge is said to headspace on the extractor. That doesn't seem to affect jacketed bullet performance a lot in the .45 ACP, since the jackets are tough enough to guide themselves straight into the throat when they are fired without deforming significantly. But cast bullets just swage into the throat at the slight angle the extractor tilts them to and part of the lead scrapes off against the chamber step, mostly blowing out after the bullet, and that destroys their accuracy.

If you seat the bullet out far enough, it will run into the throat of the rifling before the case mouth finds the step in the chamber. Meeting the throat stops the cartridge from going further forward, so the bullet and throat become the headspace determinates and the cartridge is said to be headspacing on the bullet.

See the attached illustration.

I never trim however this brings up a problem iin pistol reloading, just leave my seating die loose so can adjust for proper crimp on each round

One day long ago I trimmed a box of .44 Rem Mag brass....





Won't do that again...