My three favorite powders for the .257Robt are IMR4350, H4831, and IMR4831.
My Ruger M77MKII curiously liked IMR3031 with a 100 and 87gr bullets. I’m told that the original factory ammo was loaded with 3031. The best load through the Ruger was the Berger 115gr VLD over an ancient max load of 45.0gr IMR4831 (most newer manuals recommend lower). It trips the chrono at 3,000fps. Does ugly things to deer. Roughly MOA from the Ruger. My M98 with an E.R.Shaw barrel with 3” throat shoots close to 1/2”, though lightly pitted.
I worked up my original loads circa 1983, through my Mauser (22”bbl) and my gunsmiths Ruger#1B with a 26”bbl. the #1 liked the same loads my Mauser does.
48.0gr of H4831 (original WWII surplus) under a Hornady 100gr SPt at 2.995” yeilds 3,100fps and I once shot a 1/2” three shot clover-leaf at 200yds. Typically shoots 3/4moa. Gave just a touch under 3,200 with the #1. (non +P load, too).
My most used load, and go-to for any new 100gr bullet is the classic 45.0gr of IMR4350, Winchester brass, and a Federal 210 primer. Circa 1987, I was gifted 300rds of Federal OFB from Federal Premium ammo. I’ve still got some unused in original boxes...
Two other really good often overlooked powders are H414, and H380. H380 easily gets 3,100fps with 100gr bullet at NON-+P pressures. 3,500fps with a 75gr bullet. H414 essentially duplicates IMR4530.
Loaded to same pressures as a .25/06, the Roberts is only 75-100fps behind it with 8-10gr less powder.
Now, with all this “Bob” talk, I’ll have to take mine to the range and shoot it. Another rifle to have to clean...