I'm planning on rebarreling a British #4 action from 303 British to the 37 rimmed which is nothing more than the 303 case necked up to take 375 cast boolets. I have a copy of a line drawing given to me and it just shows a tiny shoulder. But since it will headspace off the rim no biggie. I have a older semi preturned 375 barrel blank which is marked 1x12 twist and slugs out at .377 so I'm thinking it was originally made for a 38-55. Heaviest cast bullet will be 300 grains. so this twist besides the other things may be too fast. Maybe 1x14 or even 1x16. Thanks in advance for your suggestions and comments. Frank