11mm Gras from 50-110 brass

[trenches]

It seems that I may have a weird Gras chamber. First shot out of the rifle separated the case. Comparing a dummy with the separated case you could that the shoulder had moved forward about .128". I only necked sized the brass. The unloaded cases are the same as the loaded ones. The shoulders are the same.
So I'm thinking of getting some 50-110 cases and making the 11mm Gras case out of them.
What would be the best way to go about forming the 50 down to 11mm?
I'm going to pull the bullets on the loaded cases and see if I can fireform them.
That Starline brass is awfully thin, almost like paper though, and I don't know how fire forming them will work.

[condorjohn]

I have a Gras and used fire formed .348 Win brass. Came out a little short but worked fine.

[trenches]

I bought the cases from Ammunition Antiques or is it Antique Ammunition. Maybe I should give him a call.
If his cases are correct then I have a chamber with a longer shoulder. But why?
Who knows where this barreled action came from before Century obtained it.