I have used 7mm mag. brass for years. As noted, the neck is a little shorter, but has not been a problem with 90 gr. HP or 117 gr. SP bullets.
I use H870 powder with great results.

ok savagesscout you wanted more info. so here goes.my brother had a 257 w magnum and a bunch of brand new 7mm brass that was run through a 257 die but never fire formed he bought from a friend,including dies with his favorite load written on the box.load was a 257 hornady rn soft point,with i think 66 grains of h4831 powder it was a max load,so thats what i used.sorry i can't confirm that cause he sold it and the dies.the sesults were 5 shots one ragged hole the size of a dime at 100 yrds.the gun was custom made from a mauser action and douglas barrel and fiberglass stock.