308 fired brass size

[RHux]

I loaded 70 Lapua cases with 45g of IMR 4895 with CCI BR-2 primers. The bullet is Berger's 155.5 . I re barreled a Savage using a Forster Go gauge to set headspace. My RCBS Mic measures the Go gauge at -.003. After firing ,measuring the brass with the RCBS Mic, 14 measure -.003 , 33 measure -004 , and 23 measure -.005 . How can they be smaller after firing ?

 

[dmickey]

Springback!!

 

[Martin52]

Were these new unfired cases or werer they previously fired in the previous barrel ? What case prep did you do prior to shooting them ? This will assist answers, I do agree though, springback is one reason.

Martin

 

[RHux]

This is older brass , sized in a Forster NM die with the neck reamed to 336 . So the difference in size is from different hardness of the brass from work hardening , or is it the composition of the brass alloy ?

 

[hogan]

If you call RCBS they will tell you their Precision Mic is not a calibrated tool. Meaning; each tool measures differently.

To use the Precision Mic oal thimble, measure unfired brass for oal, and then measure after firing. The Mic has no relationship to go gauge or saami specs.

Did you measure each piece of brass before & after firing? Kept notes?

How did you use the go gauge to control chamber oal? Did remove ejector (if present) from boltface? Did torque the barrel to get final clearance? Does bolt tightly close on go gauge? This is a very good way to set minimum headspace, just must be sure to eliminate variables.

Have you looked at unfired/virgin brass in your Precision Mic? The index markings on the thimble mean nothing. Markings are correct, but are not correlated to the base thimble screws onto. What the Mic measures is brass shoulder oal dimension before & after firing. Also enables you to set sizing die to keep "brass working" to minimum.

If you are shooting light loads, there could also be some inconsistency there with some brass not filling out.


Think the one variable you want to control is your brass. Use new unfired brass and measure with your Mic. If you have significant variances in length of the virgin cases, maybe try the 10 which measure shortest and size them all to same length (the shortest). Load them with a known accuracy load and test for grouping while you also record the fired length with your Mic. Might take 2-3 firing cycles to get the brass to reach max dimension so bumping shoulder back .001 while sizing yields same fired dimension.

If you have an excess of brass to sort through, maybe you find 5 or 10 of the longest and save yourself a firing or two.

If you work with one group of brass, and start with a known uniformed dimension, you should see all the brass behave very similarly.

 

[Martin52]

This is older brass , sized in a Forster NM die with the neck reamed to 336 . So the difference in size is from different hardness of the brass from work hardening , or is it the composition of the brass alloy ?

Based on the info so far, most likely the former. If the cases have been fired differing times eg some once, others 2 or 3 times obviously that will also have an impact. Try annealing then resize, that ought to bring you some consistency and give you a benchmark to help future reloading.

Martin