Ramshot shows max of around 3,100 with 60gr bullets in a 24" barrel. I would think you're max velocity would be in the 2,750 range with the 77gr.
Anyway, JBM predicts good stability with that setup down to less than 1,800 fps so I doubt it's a stability problem and you certainly wouldnt have stability problems with the 55s. They'd be OVER spinning if anything.
You used the same scope but did you CHANGE any settings?
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.