The groove dia. for a .257 Roberts is .257, twist is 1-12". The .25-20 and 250 Savage (250-3000) are both 1-14" twist and has a .256 and .257 groove dia. respectively. I cast bullets in four different moulds for these. in six different guns. Casting good bullets in small cals. is more difficult as mentioned, but good technique, equipment, metal, and attention to details, can overcome difficulties. The .25-20 I cast for do best with bullets down to 65 grains but perform best with 86 grain bullets. iIalso cast bullets in twenty -two cals. ,like for the Hornet and Bee. Now that gets much harder, the bullet base is everything , even if gas -checked. I would think that the 1-14" twist would be best for the lighter bullets, but the .25 cal. airgun shot is quite a leap from 65grain .25 cal cast bullets. The .250 Savage was always handicaped by the slower twist as Savage was trying to get the 3000fps velocity and used a 86 grain bullet to get it. The 100 grain bullets don't stabilize well in that twist even though they are better for hunting big game in that caliber. Some rifles in that cal. had 1-10" twist and seldom will give good accuracy with bullets lighter than 75 grains.
