Depending on your die or tool collection there is a way to insure proper head spacing on the sholder to began with.
IF you are starting with fired brass , do as mentioned above and make sure they will chamber and your Contender will "lock-up"
If you have a way to expand the case neck, maybe a .312 expander will be enough.
What ever you use expand the case neck all the way to the shoulder, then start running the case in a sizing die W/O the expander ball,till all but about 3/16 of the neck is sized, now start trying to chamber the case, keep lowering the die untill the case will chamber BUT requires a hard snap of the action.
What you have just done is set the shoulder and the case will head space on the shoulder.
A bit more work that just load & fire then partical or neck size and if you don't MAX out your loads it may not affect case life.