I think highly of the 257 Roberts and am of the opinion that it was handicapped until recently, when +P pressures were set by SAAMI. In my opinion, low pressures were set, not because of anything to do with the cartridge, but due to the few early Mausers, low number Springfields, that were chambered in this round. This round came out in the 1920's, pre existing rifles converted to the 257 Roberts would have been those WW1 M98's, or M96's,M91's, low number M1903's, which by today's standards, all of which were using low pressure ammunition.
WW2 and after, metallurgy was mature, process controls were at least in the vacuum tube era and cartridge pressures could be higher without compromising the structure of the firearm. Pyrometers, that is temperature gages, were an early 1900's invention, and it took time to get the things in the factories. Springfield Armory and Rock Island Armory did not have pyrometers until 1918. The US Army built 1,000,000 M1903 rifles without a temperature gage in the factory, heat was judged by the human eyeball, and as such, all one million of the "low number" receivers are highly suspect of being overheated anytime heat was applied. In 1927, Army tests showed that 1/3 of them would blow up in overpressure conditions. Also, early actions were made out of low grade plain carbon steels. By the time you get to WW2 actions are being made of alloy steels, so the same actions are stronger by virtue of better steels, and the process control technology is better.
My 257 Roberts liked the 4350 powders and shot very well with a 100 grain Sierra. I think highly of all the 4350 powders, that is IMR 4350, AA4350, and H4350. There is hardly any real difference between them so buy by price. I have gotten excellent results with 4350 in 270 Win, 6.5 X 55, and of course the 257 Roberts, and it is a very popular powder for those shooting 6.5 match cartridges. My pre 64 had the factory short magazine conversion box. Pre 64's were all a long action and the factory installed a short magazine box for the 257 Roberts. The previous owner converted it to a long action by installing a long magazine box and removing the bolt gas block stop. This allowed seating the bullet out two tenth’s longer than what I see in reloading manuals. I think this was critical for the good accuracy I got because I was able to seat the bullet 0.030” off the lands instead of having the bullet jump about a quarter inch. Check the distance to the lands and adjust accordingly. If you have a long throated chamber, I think that is just wonderful as you can push the bullet out, and pressures will be lower for the same powder charge as a short throated barrel.
Given that my loads pushed a 100 grain bullet just at 2800-2850 fps and maximum loads in my Nosler reloading manual show 3000 fps is possible, I really wonder why the 243 was created. The Roberts can do it all!
Pre 64 M70 Featherweight 257 Roberts
100 grain Sierra SP 44.0 grs IMR 4350 wtd, Lot RB 15 (60's) Fed 210S W-W brass
OAL 2.930", 2.950" touched lands.
19-Aug-13 T = 85 °F
Ave Vel = 2849
Std Dev = 27
ES = 90
Low = 2795
High = 2885
N = 10
Normal looking primers, good group except for one flyer
100 grain Sierra SP 45.0 grs H4350 wtd, Lot 22685 Fed 210S W-W brass
OAL 2.930", 2.950" touched lands.
19-Aug-13 T = 85 °F
Ave Vel = 2822
Std Dev = 14
ES = 36
Low = 2802
High = 2838
N = 5
Excellent group, under 1 MOA
100 grain Sierra SP 38.0 grs Varget wtd, Lot 4292 Fed 210S W-W brass
OAL 2.930", 2.950" touched lands.
19-Aug-13 T = 85 °F
Ave Vel = 2809
Std Dev = 12
ES = 28
Low = 2789
High = 2817
N = 5
About 3 MOA
Hodgdon's manual has loads for several bullet weights, using their relatively new Hybrid H100 V. The velocities and pressures look good. 
