I have a savage model 10 shooting 168 gr. hdy match bullets at 2700 f.p.s. that regularly shoots 1/2 inch 5 shot groups at 100 yards. I tried shooting at 850 yards and the bullets were keyholing. I moved back to 300 yards and still keyholed.Back to 100 perfect bullet holes.Back to 340 yards perfect holes. Back to 300 yards and now I get some keyholes and some good holes.I did just have a muzzlebrake installed and seating depths vary between .005 out of the lands to just touching the lands, this being the only changes made before getting the keyholes. Any ideas as to what to check? thanks trb0