The cases just need to get full length sized.
Since it's a belted mag and you're not shooting it very much, just screw your died down until it bottoms out on the shell holder. To do this you'll lower the handle on the press all the way without a piece of brass in the shell holder. Now loosen the lock ring on the full length sizing die so you can thread the die into the press with out it stopping on the lock ring. Screw the die down until it makes firm contact with the shell holder, then tighten your lock ring.
Since your cases have a belt on them the actual shoulder position isn't as critical. Your cases will headspace off of the belt (assuming that the chamber of the rifle is in good condition) and you shouldn't get any more resistance when you chamber them. Partial sizing or neck sizing are great in some situations but it doesn't sound like it's necessary for you. Your brass may not last as long by full length sizing every time, but with as little as you shoot the gun I don't think it will make any difference.
