so i have been shooting/doping my remington chambered in 338 lapua and from 100 to 425 it has been shooting great(for me) at sub moa. my 5 shot group at 400 is at 3" however when i went to 450 and 500 the group opened up drastically to about 7" from the prior groups that i have shot i feel it should do better. i know that i am not flinching as i had a buddy with me on both occasions. my set up is a stock remmy with an ior 4.5x14 scope warne rings and mark 4 base. shooting 250 SGK reloaded with 89 grains of R19 with a chronyd velocity of 2875fps. i have never shot for groups much beyond 300 yards before but have shot gongs out to 800 with my 308's. I was told that R19 is a bit of a fast burning powder for the caliber and was wondering would it possibly help to change to another? my scope is solid and this same thing has happened to me on the 2 different times i shot. shooting position was over the hood of my truck, gun supported on bags and a rear rest. sorry for the long explanation but wanted to give you pros all the information i could. thanks.