Read your loading manuals for that info.

If you get too long, you can stuff the case in the rifling and spike pressures. Short of a particular case not being able to support a bullet, there isn't really such a thing as "too short" with a bottle neck case. So if you get a tad aggressive with the trimmer, it's not a problem. Again this is for bottleneck cases, something like a pistol that headspaces on the case mouth is another ballgame; but is covered in your manuals.

 

Buy a simple caliper or case gauge as made by Lyman. Trim about .010 shorter than the drawings call for and trim when it grows by .010.
The simple Lee hand trimmers are great for single calibers and just as accurate as the high-dollar ones.

 

The simple Lee hand trimmers are great for single calibers and just as accurate as the high-dollar ones.

I have used Lee trimmers for decades without nary a hitch.

Trimming cases is about as drama filled as any issue associated with handloading. Aside from the very real need to trim noted by Darkker, there are those that trim everything, and those that never do. (Each BTW may be right).

The never do's must not own a .243 or .270, or they don't refill cases very often. Sooner than later those cases will stretch 'til the round chambers hard. In a pump action, not at all.

Much as I dislike the boring task, make that painfully boring, I trim pretty much everything.
Handgun brass can go almost forever without requiring a re-trim, especially after the first use. Uniform brass length = uniform crimp.
Some cases that require a crimp for loading, may need a re-trim every 3-5 cycles.
Some powders in some cases leave a heavy residue. The extra drag on the expander is the culprit there, and a bore brush or inside lube can reduce the need to trim as often. Carbide expanders are worth the money there.
I shot in club bench events for many years, and the serious guys felt that trimming made for more uniform case/bullet tension. I'm not sure if that's true, but they were winning, I started trimming.

Another "it depends", but trimmers sit on enough benches in some format that a lot of shooters at least think there is a benefit.

 

What caliber(s) are you reloading for?

An example 30-06: In the schematic drawing Lyman 49th reloading manual, a 30-06 case measures 2.494". That is its maximum allowable length, even though left untrimmed it will go longer. The manual calls for a "Trim To" length of 2.484". That is the standard length of a Lee Case Length Trim Gauge, if you went the route JBelk offered. If you have a different trimming tool, just set it to 2.484".

Trim anytime the brass begins to approach the maximum Trim To length of 2.494", but certainly before the next firing would make it reach or exceed that length. On most of my calibers, I will trim after each firing as a standard part of case preparation. Of course, after each trimming, you need to chamfer and debur the case mouth, so there may be some benefit to waiting longer to trim, depending on how much you brass is "growing" with each firing.

Both the maximum and Trim To lengths will be the same for any given cartridge in all reloading manuals.

 

Exactly.

It's a little confusing, but cartridge and chamber drawings use an engineering practice called a unilateral tolerance, which is done when the consequence of going outside the tolerance in one direction causes catastrophic failure or prevents assembly, while the other direction just makes sloppy fit and function, but doesn't prevent function. In that case, instead of giving a dimension that is the middle of the range and with the familiar plus and minus (±) tolerance, the dimension extreme you cannot exceed, called the critical dimension, is given on the drawing with the same total tolerance span a ± number would have in the absence of a critical dimension, but as a single number in the safe direction (plus only or minus only). In .30-06 the case length is given as 2.494"-0.020", so 2.494" is the critical dimension SAAMI thinks it could be catastrophic to exceed, while minus twenty thousandths is the tolerance. The middle of that range, -0.010", or 2.484" is usually given as the target trim length because the middle of the range gives you room for the most error in either direction when you trim.

Many pistol cases have shorter length tolerances of -0.010" or -0.006". I have personally reloaded and fired some 45 Auto cases, which shrink rather than grow with each reloading, about 50 times, at which point they were 0.025" below tolerance. They were just about all split or lost in the grass by the time that bag of 1000 had been used so much (light target loads), so I tossed the rest rather than load them again and get a lot more splits. But it did no harm. I also know people who used to figure a Lake City case was good for about four reloads in a rough M14 chamber, and would put their brass in their metals yard scrap bin after reload #4. These folks would trim them to -0.040", below SAAMI spec, when they first processed them in order not to have to trim them again before they retired them completely. Again, no problems I am aware of.

So, what's with the SAAMI -0.020" tolerance or the military -0.015" tolerance for the same brass? I don't know other than to speculate that it's because the brass specs in both instances is for all loads, and some shorter, lighter boattail bullets may not reach the minimum COL (chosen to ensure feeding from a magazine) with a short bullet in a short neck and still have adequate grip on the bullet.

For the handloader who doesn't care about the SAAMI COLs if he is loading singly or if his particular gun feeds shorter rounds just fine, it's all optional, trim length included. You just do what seems to work for you in your gun.

 

.............

In .30-06 the case length is given as 2.494"-0.020", so 2.940" is the critical dimension SAAMI thinks it could be catastrophic to exceed, while minus twenty thousandths is the tolerance. The middle of that range, -0.010", or 2.840" is usually given as the target trim length because the middle of the range gives you room for the most error in either direction when you trim.
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Nick, are there two typos here or am I not understanding something?

 

TMan, sounds like you aren't lazy enough yet. I got bored with this task to the point I shelled out money for a Giraud trimmer and haven't looked back. Man is that quick.

When I first started loading for handguns I got Lee trimmers. They wouldn't touch the shrinking .45 Auto cases, of course, but they either didn't touch or barely touched anything else. The .357 and .44 Magnum pressure range seems to be right where the cases neither shrink nor grow consistently, and I've never seen one get too long. I have, however, trimmed some of those revolver cases, not to make them safe, but to ensure crimp strength matched from one to the next.

 

I"ve got .357mag cases that have been loaded many, many times. I've never had to trim any due to being too long. Like Unclenick, I trim for uniform crimping on my loads.

 

Just know, that unless your running very high pressures or improperly sizing your brass for your chamber, you may not need to trim as often as you may think. Also, some cartridge designs stretch faster then others, again depending on pressures and proper or improper sizing.

My old RCBS calipers came with a chart providing both trim too and needs to be trimmed lengths.

It is very good practice to always keep your rifle brass sorted by brand AND number of times fired.

When brass of "X" times fired, then for that cartridge and that chamber make it a practice to always trim the brass that go into that container.

Crusty Deary Ol'Coot

 

Ah, thanks Uncle Nick and nsb,

I never trimmed .357/.44 cases until I started using jacketed bullets. When I accumulated three carbines for same. Like I've mentioned in other threads, trimming had the biggest effect on accuracy when I did the data testing and analysis using JMP/Excel. As the huge percent of the .357/.44 loads I've ever filled have used cast or swaged lead, about 95-98%, had I not bought the carbines, and scoped them, I'd still be happily loading away, never trimming.

Rifle brass, essential.

Being incredibly lazy, and at the short end of years, (joking, hopefully), I'll look at Nick's recommendation.

 

If you are using factory brass in a factory chamber there is no risk of the case being too long to start with factory chambers have plenty of clearance at the end of the neck area.
All you need to do is trim the case to the shortest case in the batch so that the shortest case cleans up nice and square and do all the rest with that setting . Slight length variations between batches is irrelevant unless it's pistol case that head spaces on the end of the case .

 

Keeping your cases trimmed is an excellent way to keep hot gas out of your face, and in some cases a chunk of steel out of your face...Be very careful to watch our case length and keep them trimmed as per Belks suggestion.

 

Neck length has almost nothing to do with pressure and I have never heard of a gun blowing up because the neck was a bit long on factory brass . In a factory chamber using factory brass it's not possible to have a case neck too long there is heaps of extra space at the end of the chamber and Sami specs prevent that from happening . I have done many experiments over the last 51 years and trimmed necks to half their original length and pressure does not rise . The only way it could be a problem is if you made a case with a neck that is so long it jams into the end of the chamber and who does that no one . Factory brass is designed by Sami specs for the mouth of the case to be way short of the end of the chamber anyway . OAL length of the loaded round is a different issue .

 

The reloading books I learned from explained why it's necessary to maintain trim too lengths on bottle neck brass, and as well handgun, both rimless and rimmed. I've never felt the need to question the advice of the publishers, but rather abide by their expert teachings.

To know when to trim is information you'll find in your reloading books, SAAMI spec. as per the cartridge.

I trim all my brass. I haven't loaded 45 acp but just a hand full of times, so don't know about the shrinking some speak of. But I have not experienced this with 9mm or .40 cal., but I have found that by the first or second trim, further trimming becomes a consistency thing. Pretty much the same thing with wheel gun brass, except that they will often exceed SAAMI max on the first firing (full house loads), after which I keep them at trim too spec. (Lee case length gauge).

Been using the Lee case length gauge trimmers for every caliber I load, great little tool for the money. One suggestion though, make sure you buy the carbide cutter or you'll find yourself dealing with a dull situation sooner than later.

SMOA

 

As little as possible and only to the "trim To" length in the manuals or .010 less then SAAMI listed length.

You think UncleNick is lazy? I paid someone else to trim them for me. It was 1K of LC 5.56 brass fully match prepped, but still....

 

What cartridge are you trimming for, LT?

The only one I've ever had fits with is the 30 Carbine, due to being a straight-walled, high-pressure round that headspaces on the case mouth. Unlike 9mm or other semi-auto cases, 30 Carbine brass DOES grow in length, which will cause them to hit the end of the chamber before the action closes.

For pretty much all bottle-necked rounds, see above, or RT*M.

 

Stretch, thanks for the typo catch. I fixed it. Sometimes my brain shuts off and the fingers keep typing. No idea why I dropped the .4 and added the zero at the end. Must have fixated on the last two digits. Country, please note the same. I dropped out that digit.

The trim vs. no-trim argument is old and based on things that aren't consistent across chamber designs. When you look at chamber drawings you see some cartridges have a sharp shoulder at the end of the case neck receiving portion, and those will pretty reliably protect you and make a case hard to chamber when it gets too long. Some of the older ones, like .30-06, have what I've always heard called a "ball" throat, which has no sharp shoulder, but rather tapers from neck diameter into the throat's shallow taper start. With these, the potential is there to wedge a long case mouth into the taper making it hard for the case mouth to expand and release the bullet. This will raise start pressure and peak pressure. I don't know if anyone has documented exactly how much.

 

With all due respect sir, when and how much to trim is one of the most basic principles of rifle reloading and if you've read one or more manuals like everybody should do before even touching a press, you shouldn't have to ask that question.
Read at least one manual (the instruction section) front to back then read it again. Reloading isn't something to be taken lightly as one small mistake or momentary distraction can be very very bad.