As far as varying powder charges and how they affect accuracy, one good test is the so-called "ladder" test. Simply put, you load a box or so of cartriges and increase the powder charge by .2 or .3 grains for each successive shell, generally staying within your manuals min and max loads. for a box of 20, this relates to a powder variation for 20 rounds of 3.8 grains for the .2 increment and 5.7 grains for the .3 increment. The theory is that you will get 1 or more groups (i.e. nodes) within the powder weight range that indicates that your guns barrel harmonics likes that powder/bullet combination and should shoot it accurately even if there is a slight powder weight variation. Some common sense is required to determine what is safe and practical. I've done it for 3 of my guns and find it to be both enjoyable and and a good way to see what a gun likes for any powder/bullet combination.
2 of the 3 guns I shot didn't seem to care how much powder was in the case. For example, my .223 with 55 Vmaxes shot a 12 shot, 1 13/16" 200 yd. group even tho the powder weight changed 2.2 grains (which is a lot for a small case, Varget by the way). My M1 Garand shot something like a 2" 100 yd group with a .3 grain variation (168 hornady match with IMR 4895).
My M96 6.5x55 did give me a vertical string about 8" high at 100 yds. which is what I would expect. Two good nodes, one of which confirmed an accuracy load a fellow shooter recommended.
So I guess the conclusion is that if your guns barrel harmonics like the bullet/powder combination of said factory load, it might shoot it just fine even with a 1 gr. variation. Ya just gotta shoot it and see.
