Re: 257 Roberts pet loads?
The loads you've mentioned don't come close to tapping the potential of the Roberts.
Drop the H380 except for bullets weighing 90 gr or less.
RL-22 is the "go to" powder for bullets weighing 100 gr and more. H4831 is a close second.
I don't know your rifle and it's personal characteristics so recommending a load isn't something I'd feel comfortable with. There is also your choice of cases, primers etc that need consideration. With that in mind, the particular loads you mentioned are very soft in any modern firearm. "Modern," means, any rifle other than some of the Mausers and other turn of the century military rifles. Possibly the Japanese WWII stuff.
A chronograph is a necessity when loading the Roberts as this is the only indicator of your rifles personality.
Target velocities for 100 gr bullets is in the 3150 fps range. 110, 115 gr bullets, right at or just below 3100 fps. Easily obtainable with H4831 and RL-22 in "good quality barrels." According to my Hodgdon books, these velocities are obtained with pressures well below 50,000 psi. Can't guarantee the accuracy of the books but I've shot a lot of these loads (mostly hotter) with no problems.
All factory loads, including the +P loads, aren't something you want to try and duplicate but you should run a box of them through the chronograph to see how close to the ammo manufacturers advertised velocities are in your rifle.