FL sizing brass to bump the shoulder .002

[blue_ridge]

I've set up my Forster 308 Match FL sizer to bump the shoulder. I also have the RCBS precision mic for 308 to check headspace. Before sizing, I'll measure cases and they will be about .001-.003 longer than Saami spec. So I run a test case through the sizer die until it gets bumped .001. Then I'll the die in small increments until the shoulder is sized .002-.003.

But for some reason, after a few cases, I always end up getting some cases that are bumped an extra .005. What am I doing wrong?

 

[Victor N TN]

Re: FL sizing brass to bump the shoulder .002

Are you looking at once fired brass? Brass doesn't reflect the true image of the chamber until it's fired 2 or 3 times. Before that the brass is blowing out into the corners and edges of both the edges of the shoulder and the neck. Basically the first 2 times you fire new brass, it gets shorter.

 

[blue_ridge]

Re: FL sizing brass to bump the shoulder .002

Maybe that is part of the problem. Right now I've got 50 cases that are between 2 and 3 times fired. They also range in OAL length between 2.012 and 2.020. So I'm trying to figure when to trim them and when to bump the shoulders. I had a couple of 3x fired cases that measured +.005 on the mic (above Saami spec) and I tried chambering them and they <span style="font-style: italic">seemed</span> a little sticky going in. But maybe I was wrong.

So do you keep firing them until they won't chamber smoothly anymore, then bump'em back?

 

[doc76251]

Re: FL sizing brass to bump the shoulder .002

There is a significant difference between trimming and bumping the shoulder back. Trimming is for keeping the brass out of the leade of the chamber which will induce overpressure of catastrophic proportions. On my old 308 the neck could grow out to 0.1" beyond SAMMI spec., it was a factory cut chamber obviously "lawyerized". On my custom chamber I can only go about 0.009" before I start getting uncomfortable.

Bumping the shoulder back as Victor mentioned isn't usually necessary in new brass for a couple of firings because the brass has not reached the chamber shoulders yet. This is one of the reasons why I advocate setting up dies for a new rifle off brass that will not chamber. You will more than likely find once you do this that your first couple of firings won't touch the shoulders of new brass. Food for thought.

Cheers,

Doc

Oh I forgot, Sinclair's has a "plug" looking thing that will allow you to accurately measure the length of the neck of your chamber. Somebody had a pic of it recently, you have to chop 1/2 the neck off a case to make it work but id does give you solid data.

 

[blue_ridge]

Re: FL sizing brass to bump the shoulder .002

I understand trimming and bumping are entirely different operations. But with a new rifle and new brass I am trying determine the proper length for each right now.

I have been looking for the tool at Sinclair since it was mentioned a few days ago in another discussion. I just found it here, under "Chamber length gauge":

http://www.sinclairintl.com/.aspx/pid=32925/Product/Sinclair_Chamber_Length_Gage

Thanks!

 

[samnev]

Re: FL sizing brass to bump the shoulder .002

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blue_ridge</div><div class="ubbcode-body">Maybe that is part of the problem. Right now I've got 50 cases that are between 2 and 3 times fired. They also range in OAL length between 2.012 and 2.020. So I'm trying to figure when to trim them and when to bump the shoulders. I had a couple of 3x fired cases that measured +.005 on the mic (above Saami spec) and I tried chambering them and they <span style="font-style: italic">seemed</span> a little sticky going in. But maybe I was wrong.

So do you keep firing them until they won't chamber smoothly anymore, then bump'em back? </div></div>

Once they are over 2.015 I would trim them as the range is 2.005 to 2.015 IIRC.

 

[427Cobra]

Re: FL sizing brass to bump the shoulder .002

You sir are overtrimming your brass, get the chamber length gauge to establish what your trim length is

 

[littlerascal56]

Re: FL sizing brass to bump the shoulder .002

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Santo</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blue_ridge</div><div class="ubbcode-body">Maybe that is part of the problem. Right now I've got 50 cases that are between 2 and 3 times fired. They also range in OAL length between 2.012 and 2.020. So I'm trying to figure when to trim them and when to bump the shoulders. I had a couple of 3x fired cases that measured +.005 on the mic (above Saami spec) and I tried chambering them and they <span style="font-style: italic">seemed</span> a little sticky going in. But maybe I was wrong.

So do you keep firing them until they won't chamber smoothly anymore, then bump'em back? </div></div>

Once they are over 2.015 I would trim them as the range is 2.005 to 2.015 IIRC. </div></div>

I use a Girard trimmer and trim to 2.010. I don't bump the shoulders (with my Redding die) until the bolt starts to show resistance when closing. I neck size (Redding bushing die) for several loadings until this happens...then it's time to bump the shoulders about .002 and check the OAL of the case.

 

[blue_ridge]

Re: FL sizing brass to bump the shoulder .002

For me, this is probably just an experience issue. (which is why I bring my questions here)

After a few firings, I measured a bunch of cases with the Precision Mic and selected the longest few (headspace) Then I tried to chamber them and it <span style="font-style: italic">seemed</span> slightly sticky. Maybe it should be more obviously hard to chamber?

Then I decided this meant the shoulders were ready to be bumped back. So I started moving the FL sizing die down in small increments. At first the shoulder actually got longer! I read this is normal because the shoulder moves up before sizing back down. Once I got the shoulder bumped back .002 or so I stopped. Good to go right?

So, I've ordered the Sinclair chamber length gauge. Will see if I can find the max length for my chamber. Then I need to fire the cases another time or two to make sure the shoulder is maxed out before I try to bump again. All that said, after 100 rounds of initial load testing, I'm average 4 shot groups at 100 yds of .350 or so. Two groups in the .1's and a handful in the .2's. My gunsmith sure can build a rifle without breaking the bank!

 

[vman]

Re: FL sizing brass to bump the shoulder .002

Yes, generally when the cases become slightly resistent to chambering (after 2 to 3 firings with no bumping back prior), you then measure, bump back 0.001, try to chamber etc etc until you find the 0.001" gap between clean easy chambering, and not so easy chambering.