Ive got a Beretta Mato in 7mm Mag that would do the same thing. After cleaning it would shoot the first 3 shot group around a inch then the next two groups would be three shots touching or damn near touching. After that it would open back up shots 9-12 would be around 2" and then it would just get worse. Didnt matter what bullet, powder or seating depth.
Mollyed up some 140 Tsx cleaned the bore to metal and laid down moly. Gun now will shoot under a inch all the time and usually better than that. Point of impact doesnt change as it gets hot. Ive shot over five hundred rounds since the last time it was cleaned.
So from my perspective I would try the moly.
IMO, if a barrel is in that kind of shape, it needs some Tubbs Final Finish polishing, as it is likely rough.
