Case length decrease ?

[relodr36]

I recently got a new AR and noticed something unusual.
The gun has fewer than 200 rounds thru it,about 90% reloads.

The cases used were once-fired WCC-93,cleaned.full-length sized in RCBS dies,trimmed to 1.7450,military primer crimp removed,deburred,Winchester primers and loaded with the Hornady 55 gr FMJBT bullet,with a light crimp in the cannelure.
Those rounds loaded with 21.4 gr of IMR 4198,show a case length increase of about 0.007" (to 1.752").
Those loaded with 24.0 gr of IMR 4895 show a case length decrease of 0.005" (to 1.740").
10 fired cases of each load were measured and the lengths were consistent - probably no more than 0.002" total variation.

????

This is my first gas gun,but I've never noticed a length decrease with a bolt or lever action.

 

[beemanbeme]

Just a guess but as a rule, semi's have a more generous chamber and your brass is blowing out to size to the larger chamber. Do some diameter measurements before and after firing.
The same event isn't uncommon with factory chambers in bolt guns. This is why I fire form before I trim to length.

 

[Larry Elliott]

Is the length decrease before or after FL sizing? If it's before sizing it just indicates that the brass moved to fill the chamber. A SAAMI minimum chamber is still larger than a SAAMI maximum case so that (hopefully) any factory ammunition will fit into any factory chamber. About the only time that a fired case won't be shorter than an unfired case is if it's fired in a very tight Benchrest chamber, and even then if sufficiently accurate measuring tools are used there will likely be a slight shortening of the case(s).

Why the rounds loaded with 4198 grew and those loaded with 4895 shrunk escapes me though.