It's been my experience and that of others using belted cases in competition that best accuracy and longest case life comes about with full length sizing dies with their neck diameters 2 to 3 thousandths smaller than a loaded round's neck diameter. Bushing dies set in the press to push fired case shoulders back a couple thousandths do this best. Sierra Bullets uses Redding bushing dies on their belted cases testing bullets for accuracy:
http://www.redding-reloading.com/onl...s-bushing-dies
When the bolt doesn't close freely an a chambered round (a little binding is felt), accuracy sufferes. The reason is the bolt doesn't close back to the same place for each shot and this tends to degrade accuracy by 1/4 to 1/2 MOA with belted cases. So a case headspace gauge (RCBS Precision Mic or equivalent) is needed to measure fired cases before they're sized then again afterwords to see how much the case shoulder's set back.
One other die is invaluable for best accuracy as well as case life. It's needed to size the little ridge in front of the belt that happens when the case is first fired. Regular full length sizing dies don't size the case body all the way back to the belt. After first using a full length sizing die, next use a belted magnum collet die set in the press to size the case body all the way to the belt. One can get 20 reloads per case using a regular die followed by this collet die on belted cases.
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