My brass is SHORTER after I fire it.

[TerroristHunter]

This is probably a dumb question, but I noticed today that my Federal .308 brass shrinks from 2.015 to around 2.009 after firing it in an Eagle Arms (Armalite) AR-10. I know most people trim to 2.005, but what does this mean?? My groups are excellent and of course in the gas gun I have to full length size for the rounds to cycle. Thanks for any help...

 

[TexIndian]

Re: My brass is SHORTER after I fire it.

If you are using new brass or brass that has been full-length sized down to minimum specs, the cases can get shorter when fired in roomy chambers. It's because the cases are spreading out in diameter so much that it 'pulls' the rim of the neck down. Just think of a tube of modeling clay. If you roll it out thinner, it will get longer. What is happening with the brass is the opposite. Then when you size it down thinner, it gets longer before you run it through your trimmer.

 

[gau17]

Re: My brass is SHORTER after I fire it.

It should grow after you full size it again.

 

[leadbullet]

Re: My brass is SHORTER after I fire it.

size it and then measure it, that is the only time a measurment matters.

 

[BuzzBoss915]

Re: My brass is SHORTER after I fire it.

What he said.

Actually, fireforming Ackley's and similar, the overall length shrinks and will never again approach factory specifications. But, <span style="text-decoration: underline">after sizing</span> is when you want to check over all length, not after firing. bb

 

[TerroristHunter]

Re: My brass is SHORTER after I fire it.

Thanks for the response. I was mainly worried about a headspace issue. It shoots REALLY good with my pet round. I use fired brass (how many times is uncertain) as long as it's at least 2.015 in length. Yesterday while developing another load, I checked myself with my tried and true cartridge and shot a .388" 4 shot group at 100yds. (Fed brass, 2.015 case, 43.4 of 4094, 168SMK and 2.800 OAL) The cold barrel shot was high. Anyway, enough bragging. My 168 AMAX round was shooting a 3" 5 shot group, so I need to break out the chrono and continue development. I think i'll shoot a shorter case and see if it stretches. When I full length size, the case grows an average of .008". Does anyone go to the lengths to order a custom die to match the chamber on a gas gun? Or is that preposterous?

 

[boltgunluvr]

Re: My brass is SHORTER after I fire it.

only way to know for sure is to size it.....since its an AR...FL size!