Heavy varmint means HEAVY BARREL.7 mag is a big game cartridge and has sort of fell by the wayside due to the cost and due to the smaller calibers 260,6.5 Creedmoor,6.5Laupa shooting beyond 1000 yards.To buy my upper new and mine is new,cost $800 or better.No offense, but your rifle is not an worth that kind of money even with scope.If you are planning on buying an 308 upper,it will not FIT a 223 LOWER.