Can someone explain how to determine case trim length?

[Cameron SS]

I know that all of the reloading manuals publish the SAAMI specs for case trim length, but is there a way to determine the ideal case length specific to MY rifle?
Are there any special tools required for this, or can one be improvised?

 

[Preacher]

Sinclair has a sintered lead plug that you insert into a trimmed case and it will give you the exact length of your chamber. A Hawkeye bore scope inserted into a opened up primer pocket will also let you know where your trim length should be...

 

[Rem06]

they make case length gauges, it looks like a .22 rimfire case inserted upside down,to measure your chamber. I think squaring your case brass is the important part.

 

[Preacher]

The part mentioned will not touch the mouth of a case....

 

[Cameron SS]

Could you explain what is meant by squaring case brass?

 

[Cameron SS]

Preacher, The sinclair website listing for the tool you mentioned indicates that sometimes specs can be out by up to .030". I don't have a bore scope and the .30 cal gauges are sold out.
How much would .030" of case brass affect neck tention or bullet pull, or bullet alignment in the chamber? Is this even worth worrying about?
I shoot 200 gr bullets which are already getting seated fairly deep into the case. I feel like a little bit more brass holding the bullet should help, but I'm new...

 

[RonAKA]

Have not done it, but I believe you can get a gunsmith to make a chamber cast and then just measure it. I would just trim it to the maximum SAAMI length.

 

[necchi]

"Cerrosafe" is a bismuth metal available with a low melt temp (190*, you use boiling water) that you can use to make your own camber casting. It's simple and instructions are available all over the net.

I struggled with this too when I began loading. sammi for 223 is 1.760,, my casting was 1.795! A wopping .035 differance! I mean that can make a differance for reaching the lands right?
Oh Man! What should I do?

Well I found out it wasn't worth the struggle, if you really look at how much .035 really is. I do trim to 1.76 and not the 1.75 they recomend.
(before I catch heat,,this is for a hunting rifle!!)
(Anal BR guy's please disregard! )

As mentioned what's more important is "squaring". That's simply being sure your trim length is straight, that there is no wobble when you trim. This assures that the neck is even where the bullet is seated then released. If the trim is crooked, then that wee bit of pressure is released on one side more than the other and your bullet won't get an even start into the leade.

 

[Preacher]

When I square cases with my Foster trimmer, I cut to the stop and then rotate the case and cut again to assure a square face on the brass... And yes it is very important that they be square at the end..
CAMERON--- Send a PM with a good mailing address and I'll send you my 308 Chamber Length gage insert, send it back when your done with it...

 

[Ackman]

I know that all of the reloading manuals publish the SAAMI specs for case trim length, but is there a way to determine the ideal case length specific to MY rifle?
Are there any special tools required for this, or can one be improvised?

The ideal case length would be within .005"- .010" of chamber length. Considering that chamber length is usually .025" - .030" longer than new brass, trimming isn't necessary or even desirable. You can find out exactly how long your chamber is with a Sinclair plug. Then decide if your brass should be trimmed. I think "trim to" length is really a minimum and to not go any shorter than that. Unless your chamber is done with a reamer spec'd to the brass, you'll never need to trim. And about the importance of having casemouths dead square.....sounds wonderful, but in real life, in the field, you'll see no measurable difference in accuracy between casemouths unsquared and squared.

 

[Cameron SS]

If the "trim-to" length is to be taken as a minimum, but trimming in general is as you say undersireable, then why bother with a trim to length at all? At the rate my brass is stretching, about .001-.0015" per firing, my brass will fail long before it ever reaches .030" longer than new. If thats pretty standard, why does every reloading manual I have talk about it as an essential step to case prep?

 

[RonAKA]

The way I understand it, the SAAMI number is the maximum for a case, and the minimum for a chamber. That way any SAAMI case should fit any chamber built to SAAMI specs as well.

 

[jonbearman]

I trim off .010 which doesnt interfere with accuracy in any of my guns. As far as seating length interfering with bullet seating,I have not had any problems with that either.I have measured factory cases and some measure .010 thou over sammi max.I think most chambers are alot more less critical than we think.The lawyers probably have something to do with this.In closing I would say trim to 1.750 and it should be fine.