Case trimming Lenngth

[GregLaROCHE]

I was wondering how much do others trim their cases to. Do people normally trim to the exact maximum length or leave a couple thousand? Does it make any difference for BP?

[Tom W.]

I trim mine to a bit below Minimum length after cleaning and sizing. They'll stretch.....

[pertnear]

I buy a case length plug for each rifle caliber I reload. I record the measurement for each individual rifle so I can monitor when the cases are getting near their maximum. When I trim, it is back to the SAAMI spec.

230346

230345

[EDG]

I measure the exact length of the chamber. I trim my cases .002 shorter the first time they are fired.
After the first firing they are resized and the cases are usually about .004 shorter. I trim all to a uniform length of about .004 to .005 shorter than the chamber. The precedeing is for formed cases that start out longer than the chamber.

For factory length brass you are kind of stuck with what you get. I trim. To a length that cleans up about 1/2 to 1/3 of the cases. Then I just let them go a while as they stretch to the chamber length. If they get really uneven I will do a minimum trim and let them stretch some more. This enables me to shoot with a lot less trimming of rifle cases.
If you need nice even cases for crimped ammo you have to trim everything the exact same length. Even this can be avoided with the Lee collet factory crimp die.

[GregLaROCHE]

I buy a case length plug for each rifle caliber I reload. I record the measurement for each individual rifle so I can monitor when the cases are getting near their maximum. When I trim, it is back to the SAAMI spec.

230346

230345

This is what I don’t understand completely. What is the SAAMI spec? Isn’t that the OAL? Is there a min max spec?

[pertnear]

This is what I don’t understand completely. What is the SAAMI spec? Isn’t that the OAL? Is there a min max spec?

Every good reloading manual should tell you the SAAMI standard case length & the Over-All-Length (OAL) which varies for the bullet being used. The OAL significance is to help with magazine feeding but there is no real standard for it. The case length is a standard & there is no min/max as part of the spec.

The danger of a too long case is that it reaches the leade & acts like a crimp against the bullet & will increases pressure.

230372

[mac60]

Every good reloading manual should tell you the SAAMI standard case length & the Over-All-Length (OAL) which varies for the bullet being used. The OAL significance is to help with magazine feeding but there is no real standard for it. The case length is a standard & there is no min/max as part of the spec.

The danger of a too long case is that it reaches the leade & acts like a crimp against the bullet & will increases pressure.

230372

Every reference for .30-06 Sprg. I have gives 2.494" as maximum case length with trim to length given as 2.484". That .010" between max. case length and trim to length applies to most - but not all cartridges.

[EDG]

You might want to go to the SAAMI web site and look at the actual SAAMI drawing for your cartridge.

The SAAMI specification list all dimensions of the cartridge and chamber. All dimensions except reference dimensions are toleranced.
By US drawing standards reference dimensions are controlled by and are calculated from other related dimensions that are toleranced.

Because of the need for manufacturing tolerance the maximum cartridge is designed to fit the minimum chamber.
If you have nominal (middle of the tolerance) the cartridge is going to be a looser fit in the chamber. So the case is normally shorter than the chamber. When that is the case there is a little room for the case to stretch without causing and unsafe condition.


This is what I don’t understand completely. What is the SAAMI spec? Isn’t that the OAL? Is there a min max spec?

[EDG]

This is the SAAMI document defining SAAMI standard cases.
Go check out the .223 Remington for example.
The max case length is listed as 1.760"-.008.
The case max is 1.760. The min is 1.752.

The chamber drawing shows the chamber length to be 1.772" +.015.
The extra chamber length is probably to enable a round to chamber with some powder residue or other crud in the chamber.
The tolerances are listed on the drawing as field notes in many cases.

https://saami.org/technical-information/cartridge-chamber-drawings/

[DocSavage]

I trim my cases .005 below SAAMI minimum specs.

[Hick]

Notice what Pertnear said-- too long a case my crimp the bullets. The point is, if the leade in your chamber is longer than standard (as some are) you can use a longer trim length, which allows for growth. In my M1, the case mouth hits the end of the chamber when the cases are 2.505, so I don't trim until they start to get over 2.500, at which point I trim them back to 2.495. This is slightly longer than the SAAMI length. My Mauser, on the other hand, is just the opposite-- I have to trim sooner. Those SAAMI specs are the standards the manufacturers work to, but each firearm is a law unto itself, due to tolerances.

[lightman]

I trim mine to the suggested trim to length. After they are fired a few times I will check them. On the cartridges that I trim on the Giraud I skip checking them and just trim them again.

[MostlyLeverGuns]

Like EDG, I measure actual chamber length, usually much greater than the length recommended in most manuals. I find the shortest case in a batch and trim all to that length, provided it is less than the actual measured chamber length. Every 3 or 4 loadings I check length, trimming for either/both uniformity and length if necessary. The book lengths do often come up much shorter than actual chamber length. The longest case that can SAFELY be chambered AND fired will usually be more accurate due to better bullet support, less jump across the gap between case neck and chamber throat.