Surprising results with Wilson Case Gage Micrometer

[RichMoAK]

After recently buying a Wilson Case Gage Micrometer I discovered some surprising results and I need some feedback.

I discovered that my 4x fired, 6 BR brass is growing by .008 with each firing. Is this normal?

In other words, I have to bump my shoulders back .008 (according to the Wilson micrometer) to get the bolt to close with mild pressure. If I bump the traditional .002 (again, according to the Wilson gage), I can't get the bolt to close at all.

I've always used a caliper/comparator combo to measure shoulder bump and have had moderate success. It always gave data that I was only bumping .002 or .003- usually based on how hard I pressed the calipers. I wanted a tool with more accuracy and repeatability. I just didn't expect this...
Are these tools just that far apart in their tolerances?

I know not to compare the data from one tool to the data to another tool, I don't understand why the case is growing so much and why only bumping it back .002 isn't enough....by a long shot.

 

[JMayo]

Humm, fired shoulder - .002 = won't fit ... doesn't make sense.
Recheck zero on gauge ?
You know jamming bullet and/or swelled .200 line also causes hard bolt.
Maybe die isnt small/tight enough to slim body down low enough.
Id bump case .002 paint with a marker, load empty case and see what is stopping it.

 

[JFrank]

I once had an issue between a Wilson case gage and a 6 br chamber, IIRC a slight difference was at the NSJ .
I don't follow a gage any longer rather use the chamber and a .300 bushing /insert to set Headspace.

 

[K22]

Something is amiss here as you suspect. If you keep truly bumping .008" that's a recipe for case head separation, it's way too much. This needs to be investigated before proceeding by someone a lot smarter than me but I'll offer some thoughts.

Does a fired case chamber without difficulty? If yes, then you're probably extruding the case during sizing. See explanation below.

Does a fired case extract without difficult without hard bolt lift? Are there any signs of high pressure such as ejector marks on the case head?

Are the cases trimmed enough to fit the chamber length? Don't just assume that the chamber is SAMMI specs. It could be shorter depending on the smith that did the barrel. A Sinclair Chamber Length gauge (less than $10) is a good investment and can give you a fairly accurate idea of the chamber length.

Are the cases bulged near the head area causing resistance to chambering?

I would suspect the gauge you're using. Do you have a traditional bump gauge and what make is it? If you truly are bumping the shoulder .002" from a fire case measurement then the case should chamber without difficulty.

But if you're not contacting the shoulder during sizing then you could be extruding the case and lengthening it. If you're using traditional bump gauges, like Sinclair, Hornady or Whidden, make sure you remove the primer before taking the fired case (unsized) measurement. Measure at least five cases.

You can also try the old method of using the rifle chamber as the gauge. This only works well if you remove the firing pin, cocking mechanism, so you can get an accurate feel of the resistance to chambering. Start with a fired case and size at "zero" cam over. Check the case in the rifle. If it chambers with no resistance, toss the case and back off the die 1/8 turn and size another case. You can start with the 1/8 turn backed off zero cam but in my experience, zero cam over does not over size a case in most presses.

Keep adjusting the die very slightly either up or down until you obtain the optimum fit. What you looking for is a die setting that will result in a sized case chambering with slight resistance, the bolt dropping down about half way. This is the way it was done before there were gauges.

There also the possibly of a faulty die that is not adequately sizing the radial dimension of the case but I think this is unlikely, at least I've never seen it.

 

[JohnKielly]

Wouldn't the obvious difference be the datum point that the new die & your old method measure at?

 

[perazziguy]

Where is Fguffey.....

 

[divingin]

Where is Fguffey.....

Off playing with his datums, I think.