I have both a Ruger M77 Mk. II and a M77 Hawkeye (neither in .260 though). It's not uncommon for a Ruger M77 to have a magazine box that is binding. Open the floor plate and attempt to wiggle the magazine box. It's should have a very small amount of play. If not, it's a
5 minute fix with a small file. Go slowly and start by removing 0.005" to 0.010" from the magazine box where it contacts the action. It can make a fair difference in the performance of the rifle.
When reassembling, pay attention to how tight you make the action screws.
When I've asked Ruger what their torque specs are for the M77 Mk. II & Hawkeye they recommended:
Front action screw: 95 in/lbs (i.e. "Farmer tight")
Rear action screw & middle screw: Hand tight and then backed off 1/8” of a turn
I don't follow that advice exactly. I have the less expensive Wheeler FAT torque wrench that only goes up to 65 in/lbs, so that's that I set the front-action screw. The rear I give 15 in/lbs and the middle I set to finger tight and backed out 1/8". Those settings have worked best on my two Rugers.
Double check the base & ring screws. IIRC, Ruger recommends the bases at ~30 in/lbs and the ring screws around 18 in/lbs. Don't over torque them and bind up the scope.
I would try that before switching powders or bullets. And a 1:9" twist is more than adequate for the bullet you are shooting.
Good luck.