zeroed my howa .243 at 100 yds with federal 75 gr nosler ballistic, and it shot under 1/2" groups. after 7 rounds i stepped out to 200 yards and it shot absolutely horrible. so i cleaned it good and shot 2 more rounds at 200 yds and it shot 3/4". cleaned, shot 2 more; same 3/4" group. i then shot 2 80 grain winchester sp's and they shot 5". i cleaned it again and shot 2 more 80 gr sp's and they hit same hole at 200. so why is when i shoot only 2-4 rounds it starts to open up that much? i was shooting in the vise and let barrel cool 3-4 minutes between shots. gun has 33 rounds through it with brand new nikon on top.