Too many things can contribute to a loss of accuracy and a rifle that was shooting under 1/2" groups normally doesn't go belly up in under 200 rounds.
Thoroughly clean the barrel. Use a copper fouling remover, Sweets, or something similar and follow the directions. Follow it up with oil and dry patches to stop the chemical reaction.
Make sure your scope base, ring screws and action screws are all torqued to spec.
I bump the shoulder on my bolt action rifle rounds -.0015" (+/- .0005") from their fired measurement. If you don't full length resize, you should consider it. Without shoulder bump gages (Hornady Lock-N-Load, RCBS micrometer, Mo DeFina micrometer, Sinclair International) it's virtually impossible to know what you are doing. Gages provide measurements that can be repeated.
Unless your magazine will allow for long loads, you will be reduced to loading rounds one at a time if you exceed COAL" by a lot. Finding an accurate load that feeds and functions from a magazine is a win/win, allowing you to make quick follow up shots.
Consider the following bullets too:
Sierra 123 grain Match King
Lapua 120 grain Scenar-L
Lapua 123 grain Scenar
Lapua 136 grain Scenar-L
Berger 130 grain (VLD, Match, Hybrid)
The lighter bullets will shoot great using Varget and RE-15 saving the H4350 for heavier bullets.
www.accurateshooter.com has a lot of load data and conversations in their forum regarding 6.5's of all types.