Stubblejumper is right - there are certain loads in certain rifles that will just throw a flier every so often - even from a machine rest. I did an extensive test back in the 1980's with a match grade AR-15. Made up a grid with all available 55 grain bullets across the top, and all suitable powders across the left column. Drew the checkerboard and triedone 5 shot group with each combination. About 400 shots later, culled out all loads that shot over .7". Took the remaining loads and fired five consecutive five-shot groups. Came up with four loads that wouldaverage under 1/2". Of these four, three would wander around a little in terms of point of impact from day to day - depending on the weather. One load shot to the same spot every time. That load was 27.5 of BL-C(2) with a 55 grain Sierra flat base spitzer. I know it was a lot of work; but don't you just love it when this happens.