Back in 1970 when a good and close friend of mine used a 7mm Rem. Mag. to win the 1000 yard Wimbledon Cup Match at the Nationals and setting a record, here's the scoop on the load. The rifle belonging to Sierra Bullets' ballistic tech, he used either new cases or fired ones full length resized with first a standard die then a body die to size the case body all the way down to the belt.....something standard full length sizing dies don't do. Back then, that's how belted cases shot the most accurate; they still do. Get a collet die from Innovative Technologies - Reloading Equipment
. It does the same thing as cutting off the bottom and top of a standard belted case full length sizing die. Use it after you use a full length bushing die; either RCBS or Redding. Sierra Bullets uses these dies to test their bullets for accuracy.
IMR 4350 is a good powder for heavier bullets in the 7 Rem Mag and there are others at that speed or a bit lower. Russian primers (from Wolf) may well be the best on the market these days.
I've used brand new Winchester belted cases to shoot 1000 yard 15-shot groups equalling or bettering current bench rest records. There's nothing wrong with Winchester brass. You won't need to prep the brass except for maybe turning the case necks to make 'em more uniform in thickness if they vary more than .001 inch.