Actually, it may very well have been the rifle. The only two things that changed were the action screws and me resting my hand on the scope to weigh down the rifle on the bags.
I think touching the rifle above the stock would have made all the difference - even if it didn't affect the way the barrel vibrated it would have affected the way it recoiled and recoil starts as soon as the bullet starts moving forward, and affects bullet departure point.
Seating position at the bench also affects point of impact - the more forward your torso is, the more effective weight behind the rifle and the more effect of the recoil movement. But I suppose accuracy should still be okay if the position is consistent, just point of impact will be slightly different compared with shooting off-hand.
