Congrats on your rifle & caliber purchase. Factory ammo for the 257 is dismal so, to make the 257 Roberts shine, it begs to be hand loaded. With 4 257s in our house I need to reload. Each gun likes a different recipe. If I had to pick one bullet to get it done , it would probably be a 100gr nosler ballistic tip. Is it a perfect bullet? Probably not but I have seen a ton of deer killed with it and it isn't a fussy bullet to reload with. Other good bullets are 100gr Nosler partitions , 100 gr barnes TTSX, 110 gr Accubonds and even a 115 gr partition. I don't think the 120gr bullets are best in a Roberts. I have tried many loads with them and burned a lot of powder & primers. Leave those 120gr to the 25-06 guys. Our 257s take a lot of deer. I have used a lot of different powders but seem to come back to H4350. Super performance, Varget, Reloader 15 & 16, H4895, Hybrid 100V , Win 760, and others. Most load recipes keep the pressures in the 45K range. I am not telling you to exceed these loads but you will find that with a modern firearm like yours, you will probably find yourself on the upper end of the velocity chart without pressure signs. As always, start low and work your loads up. Feel free to message me. I can probably send you some sampler bullets to play with.