Fire formed brass with different shoulders?

[TheGerman]

Did some load development for a hunting round out of a suppressed 16" carbine today and warmed up with some IMI 55g M193. This is from the latest batch that has reports of it being very hot and I'm going to guess that shooting while it was 110 out wasn't helping anything.

Get home, remove the primers with a universal decapper and start to measure using one of the Hornady case neck gauges and notice that almost all of them are almost identical, yet then all of a sudden I'll get a few that are .004-.006 shorter than the majority. These are all fired during the same day, same gun, same lot of IMI ammo.

What causes this? I'm suspecting it has something to do with already hot ammo plus 110 degree weather plus suppressor and it just being way overgased? Or whats going on here?

 

[MarinePMI]

Could be temp related, could also be related to how much spring back there is in the brass (how well the cases are annealed). If some cases are harder than others, they'll spring back more to the original dimension.

There could certainly be other reasons, but this may be one IME...

 

[supercorndogs]

I usually measure a handful of brass when setting up my dies because I usually find several different lengths when I start measuring shoulders.

 

[TheGerman]

I usually measure a handful of brass when setting up my dies because I usually find several different lengths when I start measuring shoulders.

Yeah I had the new barrel put onto this a while back and decided to measure for when I was going to load a batch for the new shoulder. Measured and was like WTF.

I have it on other guns as well but the shoulders were maybe .001-.002 or so different at the maximum. Was just wondering if something weird was going on.

 

[supercorndogs]

I would have to agree that .004-.006 is a lot. I am not sure if I have ever seen that much, but I have never measured the fired brass from factory ammo. I have been shooting a bunch of Wolf Gold. After the second firing a couple out of 100 showed signs of CHS. I would be willing to bet they started considerably shorter at the shoulder than the others.

 

[waveslayer]

I have had this happen before. It ended up being the original brass wasn't the same OAL. So when fire forming the head flowed a little bit differently. Thus causing different measurements