When my good friend used a borrowed 7 Rem. Mag. to win the big 1000 yard match at the Nationals in 1970 and set a record doing so, he used new cases or full length sized ones; I now forget. That incident caused more 7 Rem. Mag. chambered rifles to be sold than any other. Too bad that very few extremely accurate bullets were available at the time. But the ammo wasn't any different than the 30 caliber belted magnums used at the time.
Three rules seemed to be required for best accuracy with belted magnums (rimless bottleneck cases, too, for that matter). One was to use new cases for best accuracy. Traditional full length sizing or any form of neck only sizing didn't cut the mustard.
Another rule was that if fired cases were to be used again, they had to be full length sized in a die without an expander ball but with its neck about 2 thousandths smaller than a loaded round's neck diameter. Setting the fired case shoulder back only 2 or 3 thousandths, too. Then a second body die was used to size the case body all the way back to the belt, not 1/16th inch or so short like traditional full length sizing dies do. Innovative Technologies - Reloading Equipment
now has collet dies to do this. If one doesn't get best accuracy with full length sized cases, they ain't doing it the way that works best.
The last rule was never crimp a case mouth on cases reloaded for best accuracy. All crimping does is add another variable. It also deforms bullet jackets just enough to make 'em unbalanced when they spin at 150,000 to 200,000 rpm. All commercial match ammo as well as military match ammo for centerfire cartridges does not have crimped case mouths.