According the the 49th edition Lyman reloading manual I am to trim the 40S&W brass to .845"

I have a lot of once-fired factory brass that I am getting ready to load for. Many of the resized cases are exactly .845", however, many of the resized cases are .839", .841" etc...The Lyman manual states that since the cartridge headspaces from the mouth do not trim below the trim-to length.

My question is why would my factory brass be shorter than the trim-to length in the book, is this normal and to be expected? The brass is all Winchester, which is the same used in testing for the Lyman Manual. Do I need to throw out any brass that is shorter than the .845"?

I guess of the many thousands of rounds of 9mm 40 and 45 I never bother to measure them. I just run them through my progressive taper crimp them and shoot them.

I also have loaded thousands of rounds of straight walled pistol ammo and have never measured or trimmed a single one. The only time case length is a concern is when most brand of brass is XXX length and then you run one through the dies that has a different head stamp and it is longer or shorter. If shorter your crimp will not be correct, though not enough to upset the apple cart if just plinking. If the odd-ball case is longer than the norm the case will be bulged from the crimp and probably won't go completely into battery in the pistol and will then be hard to extract.
I think if a fella wanted the brass to be one length without variation from piece to piece he should buy Starline brass, as all of this I have used is flawless yet inexpensive.

Thank you for responding. Interesting, guess it sounds like length does not matter.

You will find that those few cases that are within 0.005" of max length will minimize head space and produce the most accurate loads.
You will find that normal pistol cases do not lengthen and never need trimming.
The only cases that can be trimmed and NOT damage accuracy are those rimmed cases, like .38 Special and almost every other revolver cartridge, that head space on the rim. Here, you are well served to trim all cases to the same length so your roll crimp will be consistent.

I think you will find that the "trim to" length of brass that you see in the manual is a guide not a rule much the same as seating depth--there will always be some tolerance to these measurements for a host of reasons. A lot of, if not a majority of, virgin brass is undersized for the same reason.

In spite of what some people think--your brass will grow with repeated sizings. However, straight wall brass does not stretch and grow at the same rate that bottle necked rifle cases do. Since a lot if not most, people do not have the equipment to anneal the case mouth of straight walled handgun cases, these cases are discarded before trimming becomes necessary. Additionally, we do not see the need for the degree of uniformity in our handgun brass that we do in rifle brass--match grade or not.

If you are shooting a 40 SW in competition, you might feel the need to segregate cases that are radically shorter than the mean case length of your lot. For general use, I do not see the need in this caliber, nor in any of the other handgun cartridges I shoot for fun.

I have never measured or trimmed a service pistol case, never, not in some 60K+ rounds loaded. The case does headspace on the case mouth, but if short, is usually held in place by the extractor. If long, the round wont chamber.

Over the years my experience with 9mm, Super, and 45 Auto is that these cases actually tend to shorten up slightly as they are fired. I shoot them until they split frankly.

Major Super gets loaded three times from new brass and then goes into the cast bullet practice pile.

Has a good source of 40 brass so I haven't run much to failure yet, but I haven't found it necessary to trim it.

Match ammo is built on new brass or fired x1 brass.

All ammo that must feed through an auto is chamber gauged.