Shoulder bumping --- need your thoughts

[Bob 964]

Beginning with my next load, I will bump the shoulder back on my brass. Until now, I have neck sized my brass. I will be using a Lee collet neck size die to size the neck and a Redding body size die to bump the shoulder. I will be using a digital caliper and Sinclair comparator body and bump gage insert to measure the amount of bump.

I measured 30 fired (from .308 FN SPR) deprimed cases at the shoulder to identify the length from which I would bump the shoulders .001 - .002". The shoulder measurements ranged from 1.544" to 1.561". This raises 2 questions:

1. Why such a wide range (.017") in measurements? I keep detailed records and measurements of just about everything, EXCEPT the number of times a case has been fired. 18 of the 30 cases have shoulder measurements between 1.556" and 1.561". Could it be that these cases have been fired more frequently than the others, so the shoulder has moved further forward?

2. Do I bump the shoulder from the longest shoulder measurement, in this case from 1.561", since I know that brass at that shoulder length will chamber?

Any insight/advice will be greatly appreciated.
Bob

 

[jdgray]

Re: Shoulder bumping --- need your thoughts

Are all the cases the same headstamp? If brass gets work hardened, that will effect the sizing for sure. I'm not sure how tight the bolt lock up is on a AR-10, seems it would need some clearance being semi-auto, and may account for some of the variation. Neck sizing only for an AR, I thought was a bit risky, and would use your body die each time. If your not getting casehead seperation, you are not to loose on your shoulder clearance.

 

[427Cobra]

Re: Shoulder bumping --- need your thoughts

First off IMHO the Sinclair HS insert sucks........ Balls, I greatly prefer the Hornady inserts.

Did you decap the brass first then measure, primer flow can sometimes lead to incorrect HSG readings, yes use the case or cases with the longest reading, this will work the brass less and make it fit the chamber perfectly, which leads to accuracy.

 

[jlow]

Re: Shoulder bumping --- need your thoughts

A couple of things. One is Jdgray is right about only neck sizing for a gas gun – a no no. You can only do this for a bolt gun. There will be reliability issues at best and at worse, I won’t go there.

Bob 964 is also right about one thing and that is you do get a pretty wide range of headspace length when you measure fired brass, even from the same rifle. I just finished measuring 82 cases from my AR-10, all from commercial loaded rounds and found a range of 0.007” in their headspace which is pretty wide. Not quite as wide as the OP’s but still.

Mine were of course all once fired being from commercial loaded rounds. The only thing I can think of is that these rounds were not that consistent and developed different chamber pressures and so expanded the brass differently. Still the question of where to bump the shoulders back to is a good one.

 

[Bob 964]

Re: Shoulder bumping --- need your thoughts

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jdgray</div><div class="ubbcode-body">Are all the cases the same headstamp? If brass gets work hardened, that will effect the sizing for sure. I'm not sure how tight the bolt lock up is on a AR-10, seems it would need some clearance being semi-auto, and may account for some of the variation. Neck sizing only for an AR, I thought was a bit risky, and would use your body die each time. If your not getting casehead seperation, you are not to loose on your shoulder clearance. </div></div>

My bad... these were from the bolt action FN SPR not the AR.

 

[Bob 964]

Re: Shoulder bumping --- need your thoughts

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 427Cobra</div><div class="ubbcode-body">First off IMHO the Sinclair HS insert sucks........ Balls, I greatly prefer the Hornady inserts.

Did you decap the brass first then measure, primer flow can sometimes lead to incorrect HSG readings, yes use the case or cases with the longest reading, this will work the brass less and make it fit the chamber perfectly, which leads to accuracy. </div></div>

Yes. Deprimed then measured. Thanks Cobra. That's what I hoped I would hear.

 

[jdgray]

Re: Shoulder bumping --- need your thoughts

If these are for a bolt gun, definately just keep neck sizing until you get a case that chambers a bit stiff. Measure this case, and start bumping a few thousands from there. Id save that case for a reference to measure from.

Good luck!

 

[afrancke]

Re: Shoulder bumping --- need your thoughts

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bob 964</div><div class="ubbcode-body">Why such a wide range (.017") in measurements?

Do I bump the shoulder from the longest shoulder measurement, in this case from 1.561", since I know that brass at that shoulder length will chamber?
</div></div>

I know the caliper + insert method seems conceptually good, but I would highly, highly recommend that for the purpose of measuring cartridge headspace you switch to the RCBS Precision Mic. Yes, it too is sensitive to user technique, but much less so than a caliper + neck insert in my opinion.

This may not eliminate the difference completely that you are measuring, but it may well shrink it down to something more readily explainable. And if it doesn't at least you can breathe easy that it's not your gear or technique and you've double-checked your measurements on the caliper with a more reliable method.

I would tend to use the longest shoulder length so long as there were a few of them in the bunch at or near that size and in general it was close to the dominant size for all of the brass I measured. I would be concerned if I were to find such a gross difference as you found, though, coming from the same gun using the same make of brass and perhaps be a bit more conservative for the first resizing, then recheck after the second firing to get the lay of the land again.

 

[Bob 964]

Re: Shoulder bumping --- need your thoughts

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jdgray</div><div class="ubbcode-body">If these are for a bolt gun, definately just keep neck sizing until you get a case that chambers a bit stiff. Measure this case, and start bumping a few thousands from there. Id save that case for a reference to measure from.

Good luck! </div></div>

Thanks JD. That's what I am planning to do.

 

[Bob 964]

Re: Shoulder bumping --- need your thoughts

Had a few minutes to kill so I measured the shoulder lengths of some Hornady cases that I was certain had been fired the same number of times. Of 40 cases measured, the shoulder measurements ranged from 1.559" to 1.562", a range of .003". This leads me to think that the .017" range I measured previously was the result of some cases having been fired more often than others, causing the case shoulder to move further forward.