Try a Lee Collet sizer. It uses a mandrel and has no expander to pull thru and distort the case. It's a neck sizer and may cause tight chambering in some guns. With the belted mags, try Innovative Technologies' body sizing collet die. It has a go/no-go gauge to determine if the case body needs sizing. The combo of these two dies can create some great shooting ammo, chambering almost as easily as full length sized rounds and a lot more concentric. Very few cases have exact wall thickness all the way around and a hard expander pull can stretch the brass, more on the thinner side. Using these two dies in concert eliminates most of that.
