Check bedding of rifle, check tension of guard screws.
Assume you are shooting over a rest, using a scope sight, correct?
Someone suggested trying factory match grade ammunition. Might be a good idea, also you might try shooting some 7.62mm Match Ammunition, military match, which you might find some of around.
Not familiar with the particular powder you mentioned. You might consider trying 4895, between 39 and 42 grains, or 3031, between 37 and perhaps 40 grains, check loading manuals.
In their 40 X Rangemaster Target Rifles, Remington used to use 37 grains of 3031, with 168 grain bullets, of unspecified manufacture. Factory target that came with mine had 3- 5 shot groups, 100 yards indoors. Average of 15 shots went into .43". I used the same load at 600 yards, and it shot quite well.