newbie question about case trimming

[timotheius]

I am loading for a .243 and the "trim to length" listed in the loading manual is 2.035. Does this mean that if I want accurate ammo I need to make sure all brass measures exactly 2.035 before I load it? After I resize my brass it has often grown to 2.038-2.044. Do I always have to trim this back to 2.035 or is there an acceptable range where it doesn't need to be trimmed?

Also, what about going the other direction? I have a batch of once fired Federal brass that all measure 2.032. Will this case length produce different results than a case that measures 2.035? Or will they both work pretty much the same?

I guess my questions is...at what point do I need to trim the brass and how far should back should I trim it?

Right now I am loading for .243 and .308. I am using fire formed brass and neck sizing only with Lee Collet dies, although I do have RCBS full length sizing dies as well.

 

[Gunpoor]

Case length is only a problem if it exceeds the max length the chamber offers, but who knows what this length is without doing a chamber cast/measurement. For the best possible accuracy the case length needs to be uniform and the case mouths need to be square. Measuring and trimming should be done after sizing because dimensions change during this process. Also, uniform length from case to case enhances accuracy, and I have seen no detriment to accuracy with longer vs. shorter as long as they are all a uniform length.

 

[varmintH8R]

Most reloading manuals list both a trim-to length and a max length. The thought process being once you get a lot of brass up around the max length, you trim them all to trim-to length.

I used to be an "every time" trimmer - making sure all loads were exactly the same for trim length. For giggles, I made up, mixed, and shot some loads that were near max and others I had trimmed to "trim-to". The net result was: nothing. They shot the same for me.

Now I only trim when my cases get long. I use my reloading OCD on other brass prep and bullet seating steps.

Bottom line- for any SAAMI chamber (all in-spec factory guns for sure) you should be fine as long as you are at listed max length or under.

 

[Kennibear]

A square neck helps when seating the bullet and keeps the runout down. I use the Lee setup and keep a cutter and lock stud with the case length gauge and shell holder for each caliber in the die box. It is not much time to touch up the necks on 20 rounds if the setup is waiting ready to go. Don't let the cases get longer than SAAMI spec as a jammed neck is very bad juju.

The sliding collar style seating dies are the best press mounted dies for low runout but I think the bullet started seating with an unsquare neck cannot help the runout.

I have a Forrester Trimmer too and it is the best but the Lee setup works better than its price. Some of the trimmers that use conventional shell holders do not hold the case head as concentric as the Forrester's collet system. They rely on the case's extractor groove and shellholder rim cut being compatible. Too much play for me. The Lee uses the flash hole to line up the case length mandrel so is not dependent on a good shell holder fit. But an off center flash hole screws thing up some.

Just my thoughts....

KB

 

[timotheius]

Thanks for all the advice so far. I really appreciate the help.

 

[AZShooter]

No need to trim often if you know the length of your chamber. If you have a custom you should be able to inquire about the reamer dimensions. If you can't do that you can measure the distance to the end of the chamber with Sinclair's chamber length plugs:

Sinclair Chamber Length Gage | Sinclair Intl

To understand the concept read this from Varmint Al's webpage:


"MEASURE THE CHAMBER LENGTH.... Purchase the Sinclair case length measuring plugs. These are very simple steel cylinders turned to the bullet diameter, leaving a rim slightly less than the case neck OD. Trim the case length about 0.100 inch short so the case mouth will not touch the rim of the plug gauge. Insert the Sinclair plug gauge as if it were a bullet, leaving it long and then chamber the case. The plug will be pushed deeper into the case neck and when you extract it, you can measure the chamber's actual length with a dial caliper. I usually find that the factory recommendations for case length leave a 0.050 inch to 0.070 inch gap between the end of the case and the actual length of the chamber. This is a factory safety concern and you can get high pressures by forcing a long case into a short chamber. The factory wants to be on the safe side. But like everything else, if you want the best accuracy, you can minimize this gap down to 0.005 inch and still be safe. "

I have found that some factory chambers are .070" or longer than SAMMI max length. I keep a reference chart in my reloading room on all my rifle's chamber lengths. Except for trueing new brass, case trimming is a rare event for a factory chamber. The custom chambers are usually shorter.

 

[Canadian Bushman]

I agree with AZ shooter
Knowing your "actual" max case length can save you lots of time and work.