I had this problem with my 30-06 Sprg. I had a gunsmith install a new Weaver mounting system along with a new Leupold M8-4x33 Duplex, and it shot great for about 150 rounds. Then my groups at 100 yds. went from .75" to a patern instead of a group, but it still grouped at 25 yds. This happened from one trip to the range to the next. We figured out the cheap Weaver rings streached and the scope was moving in them, so I replaced them with a Leupold Quick Release System, and I never had a problem again. Check your mounting system!! Good luck