Acceptable shoulder bump variation?

[CA_Shooter]

I've seen a lot of threads of shoulder bumping and how much to bump. That said, what is an acceptable or good variation in shoulder bump?

+/-.0005? +/-.0010? More?

For reference above, +/-.0005 would be, for example, 1.5400 to 1.5410

+/-.0010 would be, for example, 1.5400 to 1.5420

This would entail loading at least 80 rounds, and measuring every single piece of brass. Measuring 5 or 10 pieces would not be sufficient in this example.

I ask because I usually get about 80% within +/- .0005 and the rest are +/- .0010

Thoughts?

 

[Spoonbill]

This interests me becausewhen I measure my brass before I size, out of a hundred I might have 10 that measure. 001 above or .001 below the majority. After I resize I might have the same issue as well. I wonder if I wouldn't benefit from the Redding shellhoder set.

 

[PopCharlie]

I think -/+ .001 is reasonable expectation using a caliper and an off the self bump gauge. I like to use a bump gauge cut using the same reamer used to chamber your rifle. The Smith I use does that for me when he builds my rifles. These seem to work much better for getting a consistent measurement on your caliper. I sized 45 pieces of Lapua 6br cases yesterday. All 45 were within 1.0745 and 1.0750. I anneal after each firing. I use a Whidden bushing die in a co-ax press. I also do a 3 count at the top of each stroke.
Edit: it also is essential to develop your "wiggle" technique to get the case settled in the tool for measurement.

PopCharlie

 

[Coyotefurharvester]

Also what is the error +,- of your caliper? It is usually half a grad.

 

[1911nut]

If I got +/- 0.001” I’d be happy.

Seems I got more consistent shouder bump when I paused for a second when the ram was at full top. I learned that here.
Seems like shoulder bump measurement get more consistent after the 2nd firing.

 

[C.E.Smith043]

I agree with all of the above.

 

[K22]

I too would be thrilled with a bump variation of + or - .001".

Several variables working here, caliper repeatability, operator consistency, sizing and lubing consistency, quality of cases and work hardening of cases. Assume you are dedicating a specific group of cases to a specific rifle and rotating the use of these case to assure uniform working of the cases.

I have a single piece bump gauge, i.e. Whidden, that minimizes measuring variation. If you're using one of the two piece varieties bump gauges, place match match on the two pieces so you align them the same way every time if you're disassembling after each use and changing inserts. This can improve consistency.

One of my most accurate and consistent rifles often contains bump variation from .001 to .003" with average quality cases, i.e. Rem / Win. Yet the performance of the reloads seem unaffected - i.e. accuracy and precision is very consistent.

I tend to set my bumping up so I'm on the lower end, i.e. .001". Even if I hit zero bump the cases chamber without difficulty. What I don't want is for the case to be extruded and lengthened beyond the fired case head space or over sizing the case. I don't believe a .003" bump on a bolt rifle, while not ideal, is not excessive.

Because the full sizing die sizes both the radial and head space dimension of the case, I don't have any trouble chambering cases with zero bump. This, I believe is the advantage of full sizing over neck sizing relative to functionality i.e. the radial sizing achieved by the FL die as long as you don't extrude the case.

Also, I check my die setting for adequate bump after every 3 sizings because work hardening can require a change in die setting to obtain the desired bump. I use Skip Shims to make changes in sizing amount which is quite simple and easy.

 

[jelenko]

I think -/+ .001 is reasonable expectation using a caliper and an off the self bump gauge. I like to use a bump gauge cut using the same reamer used to chamber your rifle. The Smith I use does that for me when he builds my rifles. These seem to work much better for getting a consistent measurement on your caliper. I sized 45 pieces of Lapua 6br cases yesterday. All 45 were within 1.0745 and 1.0750. I anneal after each firing. I use a Whidden bushing die in a co-ax press. I also do a 3 count at the top of each stroke.
Edit: it also is essential to develop your "wiggle" technique to get the case settled in the tool for measurement.

PopCharlie

I've noticed that the measurement from Whidden, Mo's, RCBS can vary by 1/2 a gradation by unscrewing the top of the gauge, rotating the case and measuring again.

 

[Gargoyle]

The majority of my bumped shoulders are spot on -.002" with a handful that go -.003". I have 1/4 cases that need a second and third size to get the shoulder to desired bump setting. Once in a while I get one that sizes -.004 or more and I throw them in the trash. I figure the metal composition is off on those cases.

I stay mindful to keep lube, press stroke pressure, shell holders consistent. I run my bullets just off the lands and if that shoulder bumps too much I am reducing the bullet jump and maybe into the lands.

 

[1kbrxDave]

What does the target tell you-----------test--test--test again