Trimming bottleneck cartridges is a necessary chore, and here’s how to make it easier, and better…

rifle case trimmer
Case trimming is a needed step in the case-preparation process, and needs vary with the rifle type, and other factors. Don’t get obsessed with it, but don’t neglect it either. Not all case trimmers are equal. We’ll talk more about some I like next time, and I’ll tell you why.

At some point, now or later, bottleneck cartridges need to be trimmed. The reason is because brass flows in firing. After one or more firings and reloadings, a case will measure longer than it was when new. That extra length can only come off of the case mouth, and that’s why we trim cases. The case neck itself isn’t the main culprit in the growth, it’s just where we can address it. The most flow comes from lower down on the case.

The reason I said “now or later” is because the amount of lengthening varies from firearm to firearm and, generally, there’s usually a sooner need for trimming on a semi-auto than there will be on a bolt-action. There are two reasons for this: one is that the semi-auto will tend to expand a case more (and we’ve talked though a few reasons for that in previous articles). Another is that we’re having to full-length resize cases and set the case shoulders back a little more to ensure function. That works the brass more, no doubt. The brass is stretched more, it’s expanded and contracted more.

Important: The primary reason to trim cases is so they don’t get long enough to create a safety problem. That problem is when the case neck extends to a point where it contacts beyond its given space in the rifle chamber. That can pinch in against the bullet; excessive pressure results. The leeway will vary from chamber to chamber, and there’s no defined standard; there are plug-type gages available to measure a chamber if you want to know what you have.

case trimming
The primary reason to trim is to keep overly-long cases from overrunning their space in the chamber. If the case mouth encounters the end of its allotted space, it can pinch in on the bullet, elevating pressure. Now, there’s usually a good deal of leeway before safety can be a question, but don’t push it…

From a “performance” perspective, trimming cases should mean that all the case neck cylinders are the same height. If they’re not, then varying effective levels of bullet retention result (even if the sizing is all the same, more encasement can mean slower release).

Another is that a good trimmer will square case mouths. This is an asset to better starting alignment seating bullets and is especially and measurably noticeable using flat-base bullets.

Here’s what I do: When I get a new lot of brass, I set my trimmer so it just touches the case mouth. It takes a few tries to get this right, but the idea is that I want to see at least a skiff of a cut on each case, evidence that the trimmer contacted the case mouth. These cuts won’t all be even because not all the new case mouths will be square. Measure them all and you’ll likely see length discrepancies right off the bat. I want to eliminate those. Then I leave my trimmer set right there for future use. If we’re using the same trimmer for difference cartridges, keep a dummy case near to it and use that to reset the trimmer when there’s a tooling change. It might get expensive buying a trimmer for each cartridge you load for, but it’s sheer bliss never to have to retool a trimmer!

measuring case length
A caliper is the only tool needed to measure case length. It’s not really necessary to measure each and every case each and every time. It’s a whopping lot faster to set the trimmer so it just touches the shortest case you have (revealed through the process itself in setting up the trimmer) and trim all the cases using that setting locked in place. If it’s a fired case, make sure it’s been deprimed or the measurement won’t be accurate.

Now, there’s zero harm in using a longer “trim-to” length, and that’s way more popular than my method. These lengths are stated in reloading data manuals. Keeping up with it over years, I’ve seen no difference in the rate of lengthening trimming longer or shorter; I trim “shorter” solely as a matter of consistency over the (short) life of my semi-auto cases. Also, I trim all my cases, when I trim them. I don’t measure each case. I just trim them all. That’s overall faster and more certain.

Here’s a few things to always keep in mind about case trimming. One, and the most important in my process at least, is that the only time to trim a case accurately is after that case has been resized! That’s when there’s an accurate indication of case length. Measure a fired and un-sized case against one that’s fired and then sized, and the un-sized case very likely show a shorter length. That’s only because there’s been expansion in the case neck and body. As the expanded areas are brought back into spec by a sizing die it’s along the same lines as rolling a ball of modeling clay out on a table: it gets longer as it gets smaller in diameter.

trimmed case
A freshly trimmed case isn’t ready to go, yet, and we’ll fix all that next time too.

Also, only after sizing can we know that the case neck, case shoulder area is consistent in dimension. Measure enough of them and you’ll find some cases exhibit variance. We’re talking very small numbers here, but we’re always dealing with very small numbers, so let’s get them all the same. And that’s one of the virtues of trimming cases.

Next time more about the tools.

The preceding is a specially-adapted excerpt from Glen Zediker’s newest book Top-Grade Ammo. Visit for more information, and to order.


2 thoughts on “RELOADERS CORNER: Case Trimming”

  1. For my case trimming I have found that the LEE Case trimmer is a inexpensive, quick and accurate way to trim, I have used this setup for many years and have had no problems. This can be used in an electric drill. I obtained my various caliber tools at Midsouth Shooter’s supply.

  2. Little Crow Gun Works, Worlds Finest Trimmer 2.. Works great for multiple calibers. Been using it for 3 years now with 6 different calibers.

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