RCBS Precision Mic large variation


October 4, 2011, 01:13 PM
Hi, I've just gone through about 100 Lake City 30-06 fired in our club Garand. I found quite a difference in head space readings from -.001 under to +.009 over min. SAAMI spec. About 30% are at +.009 over minimum. Is this spread typical for military brass? Should I assume that the chamber is
+.009 over minimum?

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October 4, 2011, 01:55 PM
Bob, welcome to THR!:D

You didn't mention the source for the rounds being fired. The instructions for the RCBS precision mike state that you should be using factory loaded ammo, or at least new brass loaded for the first time. IF the LC shells were loaded by LC, then you have a good basis for your measurements.

Strange that they are all over the map. My guess is that I'd just average the readings somewhere in the middle, set you FL die for that much shoulder bump, and try to chamber them. The trial chambering should show you if they need to be set back the whole -.009. If you try .005, and they chamber but are hard to extract,(without firing of course), then you know you need to go with the .009 setting.

October 4, 2011, 03:22 PM
It can happen. Sometimes brass shortens when fired, sometimes it lengthens. Depends on a few factors including pressure, temperature, etc.

I would, in your case, size the brass to SAAMI spec of 0.000 on the mic. That's just me. I do tend to value reliability over slightly improved groups though--I'm no benchrester.

October 4, 2011, 04:09 PM
Should I assume that the chamber is +.009 over minimum?
No. Garand-type actions start opening with quite a bit of residual pressure inside the chamber. As a result,
case shoulders can/do blow forward several (many) thousandths before final extraction/ejection.

If you want to get "better" sense of the Garand's headspace, unscrew the gas plug so there is no Op Rod
movement at all upon firing, manually eject the case, and then measure it. Do this over several firings and
take an average.

October 4, 2011, 08:21 PM
The 4 Garands I've loaded for all have long chambers and shoulder expansion with my Mic shows .006-.009". I adjust my sizing die to not push the shoulder back to zero and still have the cases chamber in a couple bolt action rifles even though they are snug in the bolt rifles. I do keep a close check on brass stretching and don't plan on using the Garand brass but for 3-4 firings.

October 5, 2011, 08:26 AM
Thanks for all the replys. Will definitely try the gas plug experiment and post results.

October 5, 2011, 10:06 AM
I would determine the length of the chamber first, then form, then fire as opposed to FIRE FORMING, chamber a round, pull the trigger. It is a very short distance from purchasing a press, die and shell holder to fire forming. In the perfect world new, minimum length, first time fired cases are used. Problem for reloaders? The go-gage, no go-gage and field reject gages are fixed in length meaning a minimum length/full length sized case will chamber in the perfect go-gage length with .005 to spare, in the perfect world minimum length cases when fired in a go-gage length chamber the case will be ejected .005 longer than minimum length, .000.

Then there is the machine gun fired case, no one the length of the chamber from the bolt face to the shoulder of the chamber, no one measures the length of the case from the head of the case to itís shoulder, therefore no one knows the effect the chamber had on the case length when rejected.

If the shooter measured the cases before firing and again after firing and finds the case length has increased .004 the person doing the measuring can safely say the length of the chamber is no go-gage length. It helps to understand when measuring the new, unfired, minimum length the measurement is .000 for comparison purposes, the .009 reading includes the original .005 difference between a go-gage length chamber and a minimum length chamber, the additional .004 should be attributed to a long chamber., I form first then fire, meaning I can not adjust the chamber length so I adjust the length of the case to off set the effect the chamber will have on the case when fired.

I am a big fan of cutting down on all that case travel.

F. Guffey

October 9, 2011, 01:10 PM
Removed the gas plug and shot 5 rounds. Measured before and after. Before readings were -.003 to -.001. After readings were much more consistent than the previous results; .000 to +.002. Thanks for all the advice.

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