Both Hodgdon and Speer show 51.0 as max, so you're okay there. Large rifle, but not magnum primers, of course.
I dunno. You using a scale, or a powder measure? Maybe there's some sort of inconsistency in your weighing/measuring?
Variations in pressure can result in variations in barrel vibration, which can open up a group considerably. High, low, sideways...
My only loading of a .270 was eons ago, with a caseful of the old-style surplus 4831 compressed under a 130-grain bullet. Gave around 2,700 ft/sec or thereabouts. So, never any signs of high pressure...
You're from BATFE? Come right in! I use all your fine products!