24" groups would not be attributable to barrel heat unless there is something DRAMATIC wrong with the gun. Even if there were, there's no reason why it would show up at 200 and not at all at 100.
Something is happening with the bullets or the scope.
You should get 24" groups at 200 yards shooting from the hip!
I would guess that it's possible that the 55gr are overspun and tiny imperfections are causing instability that doesn't manifest until beyond 100 yards. Unfortunately, that doesn't explain what happens to the other bullet weights.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.