Shoulder Bump Variance?

[HK Dave]

Have you found that a variance in shoulder bump causes a big drop in accuracy? For my bolt rifles I bump the shoulder back .001-.002... however for some reason, sometimes when I bump, with the dies set exactly the same, I get a reading of a .004 bump. Has anyone found this level of variance has caused noticeable loss in accuracy?

[GeoffLinder]

This will change case internal volume and contribute to vertical stringing. But typically this type of variance is only seen way downrange, not much at under 200 yards.

Try chrono'ing your rounds with variable bump and see what the velocity diff is. Not that much velocity difference and there won't be much accuracy diff (as far as vertical spread goes anyway).

[HK Dave]

Your advice is always valued Geoff, thank you.

Next range trip, I'll mark the rounds with the variance and see if I notice a difference with the chrono.

Quite curious as to whether that tiny bit of change in internal volume would cause much of a shift.

If it ends up causing a change... I need to figure out why my resizing die is doing this.

[asm_]

The variance you noticed is caused by various level of "springyness" of the brass as the result of work hardening. If you anneal those brass that are longer than average after sizing, and run it thru the sizing die again after annealing, you'll likely to notice the you can bump the shoulder back further than the first try.

.

[HK Dave]

Even though it's all "once fired" with the same headstamp?

Although I'm sure a lot of it was made at different times and probably had different lot numbers.

[GeoffLinder]

Different lots of same mgfr can have different qualities, but usually a single mfgr is pretty close compared to other brands.

If it was shot in rifles with different chambers, the amount of bump needed will likely cause some small amount of variance in sizing result.

Are you using a gauge to measure the bump variance or just checking OAL?

[HK Dave]

Using a gauge specifically designed to measure the bump variance.

I think that must be it... it must be because they were shot on different rifles.

Guess I better further sort according to the rifle it was shot out of to get better consistency... or i could just go buy some virgin winchester brass and be done with it.

I hear though that virgin brass may need the neck stretched out? hehe

[LynnJr]

You shouldn't be seeing 0,.004 variance.
Are the spent primers still in the cases as this will give you that much variation.
I just did 700 pieces of Norma 300 WSM brass with 1,2,3 and 4 firings on it and never got that much variation.
The problem with 0.004 variation is your neck lengths won't be the same and the shorter rounds will have a flatter pressure curve.
If you don't have that many short cases I would create a false shoulder and re-fire them.
Lynn

[GeoffLinder]

For plinking grade, blaster ammo, I would not sweat this. For top quality ammo using high zoot match grade bullets, one batch, weight segregated cases fire formed to your rifle to start with are always the best ticket.

BTW, if you let it ride for the moment and go and fire all these cases once through your rifle, I will bet they bump more consistently next time around.

[ar15barrels]

Dave-

Seek out my posts on "hard bump" vs. "soft bump".
You are likely soft bumping the shoulders and inconsistent brass hardness is causing your length variation.

[problemchild]

Variance also comes from "press stretch" when the press is loaded up on all stations. My press will stretch .005 when loaded up.

Yeah it makes a difference on some guns. My lmt likes .002-.003 bump.

[hanover67]

I reload .308 for a match rifle and over the years I have had a lot of brass that has a different shoulder length after resizing. I attributed it to differences in the brass (springiness), especially with Lake City Match brass. I never noticed any accuracy issues. I was more concerned that they would cycle properly during rapid fire. So, I always full-length size rapid fire brass, but only neck size it for slow fire. I have some old WCC 70 cases that seem to be very accurate, even after many reloads, that I use for 600 yards.

[HK Dave]

Randall thanks brother... found this that you wrote...

"Shoulder length variation is due to two factors.
First factor is brass hardness.
Second factor is press frame stretch.

There are two different ways to adjust your shoulder length.
I call them soft bumping and hard bumping.

Soft bumping is using a regular shellholder and adjusting the die down until the case is getting sized enough.
This is how 99% of people do it.
The issue with soft bumping is that the press frame stretches different amount based on the differences in brass hardness.
So, harder brass stretches the press more and the shoulders do not size down as much.

Then we have hard bumping.
You will need shims or redding comp shellholders to do this.
Hard bumping is when the die and the shellholder are in FULL CONTACT during the sizing of every case.
I call it hard bumping because you will feel a more defined "bump" as the press cams over.
The bump will be the same regardless of wether you are sizing a case or not.
With hard bumping, you are letting the die and the shellholder control the shoulder location, NOT the brass.
No matter how soft or hard the brass is, the shoulder lengths are consistent.
The redding comp shellholders come as a set of 5 different lengths.
Each one is 0.002" longer than the one below it.
You start by adjusting the press for a hard bump.
All you need to do is size a case and adjust the press so that there is no light between the die and the shellholder while the ram is at the top of the stroke.
Then add another 1/8 turn to the die.
This will give you excessive headspace with most dies.
Using a shoulder length measurement tool, determine how much longer you need the shoulders to be and swap in the proper shellholder from the redding set.
The redding comp shellholders are all the same length, but they are different internal depths from the top of the shellholder so the spot where the case head sits."

So I must be using my resizing die incorrectly. Will try to use it the way you stated and "hard bump" my brass from now on.

I just measured all 200 cases I did... sob, there's as much as a .005 difference between some cases. I know for sure none of my rifles needed that much of a bump.

I'm guessing the only way to reduce the bump is to fire form?

[HK Dave]

Oh and for precision match reloads... do you think i'll notice a difference in accuracy with a .001 neck bump variance between brass?

I am trying to keep everything else as uniform as possible from brass length, to OAL, to brass weight etc.

I ask because I am about to run a ladder test at 300 yards and am hoping this doesn't cause false results.

[ar15barrels]

So I must be using my resizing die incorrectly. Will try to use it the way you stated and "hard bump" my brass from now on.

You will need to make shims or get the redding comp shell holders to hard bump.
Simply pushing the shoulders all the way down as far as your die will allow is only going to get you very consistently excessive headspace.