I know this is easy but.....case length and trimming...

[rsbhunter]

Ok, new brass...say it measures 1.500 oal (example)...info says max is 1.505...so, after 1 st firing, datum on shoulder measures .010 longer than new brass, and over all moves to 1.510......now, if for being safe, you bump the shoulder back.005, does that not also shorten the case oal ? I know I'm over thinking this, but I can't figure whether to trim resized case or fired case to "trim to length" , or what ever length I choose is good to go....Thanks for any help, rsbhunter

 

[Mauser06]

Resize and then trim to length.


How often? You be the judge. Personally, I trim, chamfer and debur every firing just for consistency. Trimmer just barely kisses 98% ofnthe cases. Always a few that seem to have a bit extra taken off.

 

[SSL]

Trimming should be done on sized cases, so the length of the fired and trimmed case should determine the issue.

 

[Dub]

Ok, new brass...say it measures 1.500 oal (example)...info says max is 1.505...so, after 1 st firing, datum on shoulder measures .010 longer than new brass, and over all moves to 1.510......now, if for being safe, you bump the shoulder back.005, does that not also shorten the case oal ? I know I'm over thinking this, but I can't figure whether to trim resized case or fired case to "trim to length" , or what ever length I choose is good to go....Thanks for any help, rsbhunter

actually it is a very good question

 

[D-4297]

Suggest you re-size case first , then check over-all length . On first firing ; some cases will grow only at the shoulder , as it "sizes - out" to fit your chamber . And chambers usually have a amount of extra clearance for neck length growth . I trim my .308 cases to 2.010" knowing I have another .025 clearance to the Leade . A few may grow .005 but not many . I think I'd wait to trim till at least the second firing . I just went through the 450 cases I shot at the FCNC , and found 20 that had grown at the OAL .003 - .005 after resizing on the first firing . Lapua SRP Palma cases . But always resize before trimming . first load , or tenth reload .

I just caught something in the OP's comment . I also wouldn't Bump Back .005 on any case . That is considered excessive . Measure the current shoulder Datum on several cases and avg. that number out . If you're shooting a Bolt gun , Bump .002 from the average datum number an check the case for fit in your chamber . If it drops in , and you can close the Bolt without excess force , you're good there . Over-working brass shortens it's life considerably .

 

[rsbhunter]

Thank you for all the replies, it has helped me wrap my brain around it...actually, these are for an AR in 6mm ARC. We shoot a match at 4"x4" steel plates at 200 meters, 5" x5" at 300 meters, etc. out to 500 meters. 10 shots in a minute and a half.....so any stoppages are very bad juju. My Dasher I set back .0015-.002. Please know I welcome any advice or comments... I am always learning, or trying to...rsbhunter

 

[Sniper338]

Have you measured your chamer to see what max is?

 

[rsbhunter]

No, it's a SAMMI chamber, which means nothing I understand, but I'm going by cartridge drawings. I'm not even worried about running them to long, was just thinking about the factors that might influence mistakes in oal case length....more of a ?.....rsbhunter

 

[mikecr]

It's good to go ahead and measure chamber length.
As long as you have reasonable clearance with NEW brass to chamber you should be ok to defer trimming for later. 1st fireforming often pulls necks back some as shoulders expand.
Then I watch it, especially within 10thou of end, and trim to 5thou as I can.
I shoulder bump 1thou, and I don't FL size, so it's been a long time since I've needed to trim anything.

In my view, trimming is just one of those things that some people overdo.

 

[rsbhunter]

So far, all loads shot are in new brass...Hornady 105gr Black , and Star line new brass...so still in the forming stage. I did have .010 growth on base to datum line on shoulder.... Was a little surprised. As far as setback goes, as stated, function trumps razor edge accuracy... Will see what happens with loads loaded in fire formed cases...rsbhunter

 

[Walt Krafft]

When loading an AR you want to make sure ALL cases are trimmed short enough. Unlike a bolt gun where you could feel the resistance, the bolt of an AR will slam shut on a long case and crimp the bullet, making pressures go sky high. I found it was not good enough to just sample a hand full of cases to check oal.

 

[LCazador]

So far, all loads shot are in new brass...Hornady 105gr Black , and Star line new brass...so still in the forming stage. I did have .010 growth on base to datum line on shoulder.... Was a little surprised. As far as setback goes, as stated, function trumps razor edge accuracy... Will see what happens with loads loaded in fire formed cases...rsbhunter

This is typical of new brass where it will be as much as .015" under actual chamber at the base to datum measurement. So in your case where it that measurement grew to .010" that should be within a .001" of your actual chamber measurement at the base to datum. You should be bumping .001-002" off this measurement for proper sizing. A bump of .005" would be far too much but every rifle is different. After you do this bump then you would measure case length and make sure you less than the maximum recommended case trim length of 1.505". With autos I prefer a .003" bump!

 

[rsbhunter]

I agree with a .003 bump...there's alot of violent movement going on in a very short amount of time in an AR. I honestly believe it's a trick to get 100% function and great accuracy out of a caliber bigger than 224 in an AR. I have seen some very accurate ones, but it a long trip down that rabbit hole!