I have new brass...

I have a mauser I just built with a new 35W barrel, and every thing is up to spec, IE headspace, firing pin protrusion but apparently the
chamber is a bit sloppy and if I use a new piece of brass occasionally I get a FTF. With a primer in the case only, then fire - the firing pin is setting back the shoulder about .013" or so.

I posted the problem on the mauser forum it was said the small shoulder and loose chamber fit was the culprit and the best way to fix the problem would be to fire form the brass then neck size it, then with a better chamber fit, it should resolve the FTF problem.