Sometime around 1984 The American Rifleman published some very much reduced loads for deer rifles. The idea was to use your deer rifle for squirrel hunting. I loaded 9 grains of Unique with 100 grain bullets for my .270. Shot well and worked on squirrels. A buddy loaded something similar for his .22-250.

I've lost the article. Does anyone have a load like that for the .257 Roberts?

Have you looked at Trail Boss?

http://www.imrpowder.com/PDF/Trail-Boss-data.pdf

Many states do not allow center fires for squirrels; make sure you have good backstop; those heavy bullets will carry much farther than a .22LR. If you used 9 grains of Unique in your 270, you already have a base line to work from.

I have used mine for ground squirrels and coyotes quite a bit. Mostly used Hornady 75gr with IMR 4064 but they weren't reduced loads at all.
I also tried the 60gr pill for the .25-20 and it was zooming but did way to much damage to fur and wasn't as accurate.

I've used 90 grain cast bullets over, I think, 10 or 11 grains of Unique in my 257 for squirrels. I was trying for a 25 rimfire equivalent. Another option would be to try a No. 3 buckshot over 5 grains of Unique. Or as I mentioned in another thread, take a look at a Hammond Game Getter. They are pretty accurate to 25 yards and cheap to shoot with across the counter components.
Bfly
Bfly

The old "basement" load of this nature, you take buckshot (used to be common and cheap. Maybe common but no longer cheap. 000 is about .36, 00 is about .32, #4 is around .25, regardless you can squeeze it down or bump it up without much effort...) and load over a small charge of anything suitable. On top you dab lube. Parrifin? Beeswax? Crisco? Lard? Etc. Of course you get the most loads from, say, Bullseye. Tiny charge and increase it to the use. Inside, not much. (Or just a primer.) As distance increases you will be unsatisfied with the drop and failing accuracy and ready for a cast bullet with the kind of tested load Lyman lists in the books. Then you buy a mold. Luck. Happy Trails.

LOL, tried a reduced load in my 7.65x53 1891 Argentine when a young guy back in the late 1960s, for squirrel hunting. The old Argentine was a good deer rifle for me. Recall using 13 gr of Unique and around a 100 gr bullet back then.

Well, shot at a target on a tree to test the load and went off to hunt squirrels. Found one. Twelve shots later, the squirrel was sitting on a low limb of a large oak tree eating an acorn, basically laughing at me while he ate. Out of ammo, I actually looked around for a rock to throw at him. I'd played a lot of baseball and was a pitcher with darn good control.....would have had better luck hitting the squirrel with a rock, than what I'd done with the reduced loads.

No rocks to be found. So, took the FN 9mm hi-power pistol, that I always carried back then, and fired what I thought would be one of many rounds from its 13 rd magazine. Darn if I didn't hit the squirrel dead center between its eyes. I couldn't hit a barn with the hi-power.

Anyway, give the reduced loads a try. I've had good luck at range with reduced loads with IMR4759 and IMR trail boss when shooting targets. Should work.