R/C - Just FYI, my M14 shoots 150 grain SST into 5" shotgun patterns, and 165's into nice 1.5" groups, with the same brass/primer/powder types. I'm sure it's the short bearing surface of the 150's. Flat base Winchester 180 PP's shoot even better. I know you've heard about the not shooting 180's thing, but I use the same powder (IMR 4895) for all my M14 loads, just adjusting how much I use, and never had a problem.
I'll be hunting my moose this year with my M14 and the aforementioned Winchester PP's as I have about 40 left loaded and I'm out of 165 grain SST's right now.