I have two problems bench shooting. The first record group is generally larger than the next four, and from the sixth group on groups get bigger.
I shoot 10 groups, slow and as carefully as possible, don't clean the gun until I get home. I thought that maybe the gun needed cleaning between sets of five groups.
I shot 1 through 4, then after one shot on the top right, a target change was called. After that, I went to 5 and 6, they were bigger and had moved south., I cleaned the rifle, 2 foulers, 7 was still big and low, 8 and 9 were bigger and low. It didn't seem that cleaning helped.
It was the sun. Shooting early and slow, no sun on the barrel. After ~ 8AM, the sun shines on the barrel, barrel gets hot, groups move south and get bigger. The barrel was hot to the touch by 9AM. This only took a year to figure out. I'll confirm it next time. joe b.