Most belted cases like the .300 Win Mag shoot most accurate when they headspace on their shoulder. That better aligns the case neck and bullet with the bore.
However, folks handloading new belted cases seating bullets to set back a few thousandths by crunching into the lands when chambered have got sub 5/8 MOA accuracy at long range with new cases. This is what the military teams did with the .30-.338 back in the '60's when normal full length sizing of fired cases didn't do so well. Even when setting the shoulder back a couple thousandths. So they just used new cases tossing them to civilians when the match was over.
One other thing about belted cases. The ridge that builds up right in front of the belt has to be sized down else it'll interfere with repeatable case positioning at its back end. Full length sizing dies do not size the case all the way to the belt; they stop 1/32 to 1/16 inch short. A collet die's needed such as the one sold at Innovative Technologies - Reloading Equipment
. Before that colled die was offered, folks cut the shoulder off a belted case full length sizing die as well as the bottom just above the belt clearance. Squared both ends then slightly radiused the bottom part. Set that die in the press such that the case stopped just as the belt touched that die. Best accuracy with reloaded belted cases ever; other than new cases.