View Single Post
Old October 8, 2012, 10:10 PM   #16
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,321
Normally, when a bottleneck case is fired, it gets shorter in overall length; the brass gets sucked back into the chamber as the case expands. Neck only sizing lengthens the neck (and therefore the case) a thousandth or so, but full length sizing lengthens the case more; the brass has to go someplace having been reduced in diameter. Having full length sized fired cases with both standard dies and dies not needing expander balls (same body dimensions in each die), how much they get longer with both methods is insignificant. This happens with both rimless and belted cases that headspace on their shoulders. New belted cases headspacing on their belts may get longer when fired if there's enough shoulder clearance when fired to allow it when the belt's hard against the chamber ridge and the shoulder's pushed forward against the chamber shoulder when fired.

Depending on how much the differences in dimensions of the chamber and case shortening amounts will differ when its fired. Measure case length before firing, after firing and again after sizing it to see how much its overall length changes. The more a fired case is sized with the die, the longer it gets.

In a case life test some years ago, I reloaded the same Federal .308 Win. brass case 47 times before running out of test powder. A full length sizing die was used with its neck lapped out to .335"; a couple thousandths smaller than a loaded round's neck diameter. The new, virgin case was trimmed to 2.000" when first loaded with a Fed. 210 primer, 42 grains of 4895 under a Sierra 165-gr. SBT bullet.

After its first firing, the case shortened about 4 thousandths. Full length sizing it to reduce body diameters about 2 thousandths an set the shoulder back the same amount, case length grew about 5 thousandths to just under 2.001". After 10 reloads, that case had grew to about 2.008" when it was trimmed back to 2.000". 10 reloads later, it had grew back to about 2.008". Same thing for the next 20 reloads. After the 47th time, it had grown to about 2.006". That case was never annealed.

A friend did the same test and his Federal .308 Win. case went 56 reloads before he ran out of IMR4064 under Sierra 168's. His load was fired in a match rifle clamped in a machine rest and put all shots in 1/3 inch at 100 yards. He didn't anneal his case either. His case had about the same case length change issues as he used the same type of full length sizing die as I did; I had lapped its neck out for him the same as my die had been modified.

Last edited by Bart B.; October 9, 2012 at 08:32 AM.
Bart B. is offline  
 
Page generated in 0.04777 seconds with 7 queries