SE, folks shooting belted cases back in the '50's and '60's learned that the first time a belted case is fired, it typically gets a tiny ridge about 1/32 inch in front of the belt. That's where the case body in front of the belt expanded against the chamber headspace shoulder edge. Unless that ridge gets sized back down to the rest of the body diameter, best accuracy won't be had. It causes inconsistant positioning of the back end of the case in the chamber.
We used to cut the middle section out of a standard belted case full length sizing die square up its bottom then set that in the press to size a previously full length sized case all the way to the belt. Standard dies stop about 1/16 inch or more forward of the belt. If that ridge ain't sized back down, it'll interfere with perfect case positioning at the back of the chamber. That'll make the barreled action not wiggle and whip the same for each shot.
Nowadays, a collet die's available to do that. www.larrywillis.com
is a site that has them. Read the stuff about this die; good to know.