I don't know if this is common or not. I bought 280 brass, loaded it up, shot to fireform to ackley. I trimmed all cases to spec before loading so after firing and forming the brass went from 2.54 to 2.515.

I contacted the gunsmith, he said it was ok, and that the cases will stretch with more shots. Is this true? This is my first time fire forming so I don't know what to expect. Gunsmith also said I can trim them a little shorter?

AI did the same for me, you will not have to worry about trimming for a while.....after you true them up, just go shoot.

Quite normal, nothing to worry about.Rick.

Normal. Go shoot and enjoy not trimming.

Not trimming? Hmm, I'm trying to see a problem here...

Why did you trim them before initial shooting?

Ted

Don't trim any cases until they get over maximum trim length. I know you probably wanted to trim them all to a uniform length, but I haven't noticed a difference in accuracy in cases with varying lengths, so I just trim them when they get too long now.

There is also nothing wrong with coming up with your own trim lenght based on your brass and what they are now. You could just go slightly longer than the shortest one, and trim them all uniform. Then if you are crimping or seating to a cannelure, they will all match. All long as you are not over the max case length for your chamber, they will be fine. The tiny difference in amount of neck grip or whatever means nothing.

Of course they're gonna get shorter. Blowing out the shoulders has got to change something. If you were pushing the shoulders in, it'd make the brass longer. Usually when you 1X any brass it will come out shorter.
And I agree that it's pointless to trim brass-any brass- before fire forming it.

Thanks everyone
I would say there is definitely a consensus that it's normal.

The trimming was because I started with 280 brass, wanted them uniform, didn't know that it didn't matter because the case was going to shorten. I'm learning. New to the fire forming world.