Shoulder Bump Measurement Accuracy

[Grandmasta J]

So, I've been measuring shoulder bump on my .308 brass using a Hornady headspace comparator and, based on those measurements I thought that I had finally got my full-length sizing die set correctly for a .001" shoulder bump... BUT, I pulled out a Sinclair bump gauge and got a completely different reading! Before anyone says anything I realize neither of these gauges measure actual headspace, they just give you a measurement to compare another measurement against but, the problem is I measured the exact same brass and the Hornady gauge told me I was getting a .001" bump and the Sinclair told me .006"!

Here are the exact measurements:

Hornady Gauge: Fired brass - 1.626", Sized brass - 1.625"

Sinclair Gauge: fired brass - 1.575", sized brass - 1.569"

So, clearly these gauges measure from different places on the shoulder, and clearly one is more accurate than the other, the question is - which one???

 

[jglenn]

Most bump gauges measure off the datum line. The sinclair does not. It measures off the shoulder. No offense to Hornady but I'd opt for the Sinclair based on their history alone. Reading off the shoulder is a bit easier I would think. Try one and stay with it for a bit and see how it goes. The gauge I use measures off the shoulder but use a dial indicator.

 

[Tracer847]

I have had good results by finding a case that hard to close the bolt on (can’t find one …make one by neck sizing until you get one). Then creep the die down until the bolt closes with little effort.

 

[Grandmasta J]

Most bump gauges measure off the datum line. The sinclair does not. It measures off the shoulder. No offense to Hornady but I'd opt for the Sinclair based on their history alone. Reading off the shoulder is a bit easier I would think. Try one and stay with it for a bit and see how it goes. The gauge I use measures off the shoulder but use a dial indicator.

Yeah, I agree the Sinclair is nice but, I decided to stick with the Hornady because I'm not interested in measuring off of the intersection of the shoulder and the body, while that may be useful for prolonging the life of your brass, I'm more interested in measuring the part of the shoulder that actually stops forward movement of the cartridge - otherwise you're essentially changing the seating depth each time

 

[Grandmasta J]

I have had good results by finding a case that hard to close the bolt on (can’t find one …make one by neck sizing until you get one). Then creep the die down until the bolt closes with little effort.

Yes! When I get time, I plan on taking the firing pin and ejector out of my bolt to see which sized cases have little to no resistance - ideally the bolt will close on the case om its own. That seems like the only way to know 100%. From there I'll use my headspace gauge to get the measurement for future use.