[Clark]

If I fire form 257 Roberts Ackley Improved with 18 gr Bullseye and a bullet, the case shrinks in length. If I increase the charge, the case stops shrinking, but gets cracked.

Now I am fireforming with 10 gr Unique and cream of wheat. Now I only get 90% of the shoulder filled out, but no shrinkage, and it gets filled out when I fire a bullet.

[KP95DAO]

I don't know if you are using factory cases or what. You might try using 7mm Mauser cases and neck them down using the unsized portion of the neck to establish solid headspace. You could then use a 90% charge to form your cases.

You might have to trim the cases prior to loading; but, you would have to do that anyway.

[Steve Smith]

Maybe try annealing it before the fireform?

[Clark]

Those are good suggestions.

I have been using new Win 257 Roberts +P brass.

I have not annealed any brass for this caliber, and maybe I should.

I have some old Wilson vise dies in 7mm and 257 Roberts that easily make 8mm brass into 257.

[swifter]

Mr. Smith has it! In forming my AIs, I routinely anneal before fireforming. Its improved my success rate enormously...:D

I'd also suggest you save the COW for breakfast, use a bullet, and a close-to-max load with a quickish powder for the parent case. I believe the COW is abrasive to barrel steel, and can act, sometimes, as a bore obstruction.
There's also some evidence that brass has a "memory" and it is important to form it right the first time.
My dos centavos...:eek:
Tom