Re: 260 Loads?
140 A-Max or 142SMK, 43.8-44.0gr of H4350. The 140 worked when i started and teh 139 Scenar did as well, but teh 142SMK is superior to teh 140SMK, and the Scenar tended to generate more pressure for a given load. My initial efforts with the 139 were on the hot side, and I backed away from it when I started getting good results from the 142SMK.
For Varmints try 95gr V-Max and 47.0gr (published minimum) of H4350. You can probably go faster, up to a published max of 49.7gr of H4350, but the 1:8" twist may cause problems. You'll have to try it, paying attention to pressure, and see if the bullets hold together at the higher velocities.
I found plenty of velocity and good accuracy at the 47.0gr load level, and never tried for more speed. I tried a barrel tuner with this bullet, and 200yd accuracy was excellent once a sweet spot was determined, but the sweet spot kept changing with the temperatures.
I don't know how the 95 V-Max handles extreme velocities in a 1:8"twist, and with my barrel being 28" long, I'm probably already in the 3300-3400fps range.
I would suspect that Hornady made the bullet tougher, knowing what it might be up against, but I don't know yet for certain. That may be a project for this season.
With components being on the iffy side these days, trying out the 123SMK may want so sit on hold for a while.
Greg