Darn, I just lost my last reply, so here goes again.
In my last reply I said that 40 grains of IMR4895 shot very well in my DPMS LR-308, heavy barrel, 24 inch. I have two identical uppers, scopes and all, set up exactly the same, so it is just the two pins to swap them, and I also have wedges cut to make a tight fit in the lower halves. There is about .004 difference in the wedges between the two uppers, so I have to make sure what wedge I use, or I can't close the halves together if I have the wrong wedge. I do have a Geiselle national match trigger, without which I would be unable to get the groups I do.
40 grains of 4895, or 39 grains of varget gets 2450 fps at 10 feet in a 24 inch barrel, with Sierra 168 grains. It is when you go for 10 shot groups is when the men get separated from the boys, HI HI HI. I have found that in my rifle, if I go up to 2550-2600 fps, my groups will begin to open up vertically. My 10 shot groups are consistantly 5/8" at 100 yards with the 40 grains 4895 load, and BR-2 primers. If you shoot the Sierra's at 200 yards, you will find that the group sizes don't grow much over the 100 yard groups. It seems like the 168 grain match HPBT settles down better out there. I know my DPMS at 300 yards, the groups are 1-2" when I shoot at a range at Salmon, Idaho, in the morning with little wind blowing.
Also, try Federal 210M or CCI BR-2 primers to see if they make a difference in group size. I have lots of each.
Realistically, a 10 shot 1/4" group in a semi, or a bolt for that matter, will be very tough, especially if you are shooting 100 yards or more outdoors. I can easily get 5 shot 1/2" or better groups. (witnessed targets)
The groups I shot yesterday were very good, but the wind changes direction at the range 3 times (flags) in 100 yards.
At this range, at Issaquah, Washington, it is in sort of a valley, created by overhead barriers, and trees, which can give wind shifts up to 4 times from the bench to the 100 yard target frames. Still, I got 5/8" measured 10 shot groups, if I did my job.
2 years ago, 74 degrees, no wind, I shot 2-10 shot groups at 100 yards with a 17 HMR 17 grain bullets and 20 grain bullets, that you could cover with a dime. (witnessed).
So don't give up, but if you can get 10 shot groups of 1/4" at 100 yards with a semi, (witnessed) I will take my hat off to you.