Could the members here help with a 37 rimmed cartridge which is nothing more than the 303 British cartridge case with the case neck expanded from 303 to 375 and no other changes made?. Looking for a twist rate on a new barrel in 375 caliber for up to 300 grain cast bullets. have an old semi turned barrel blank with a 1x12 twist but that seems to fast for cast bullets. Was thinking about either 1x14 or 1x16. My 9.3x57 mauser has a 1x14 twist with the 286 grain bullets. So was thinking a slower twist is needed. Thanks in advance for any comments and suggestions. Frank

The twist rate for the smaller 375 Win was 1 in 12" and for reference the similar 38/55 is 1 in 18". The more recent LR 38/55 single shot barrels for launching bullets weighing well over 300gr are also 1 in 12".

I have a 375 Win and it works really well with cast 300 gr cast pills. I'd be more concerned about the throat and bullet fit than the twist rate.

That 12 twist will be just fine your your bullet.

The 375 barrel with the 1x12 twist already had some work done on it prior to me getting it.And was very cheap!!. Will start out with that one as if I mess up I'm not into it for much money.While not a true cast bullet rifle I do have some boxes of Speer 235 grain I'd love to shoot and see how they perform. So maybe a 1x12 twist might work out after all. Frank

Thank you all for your information. I'm starting to think of the 37 Rimmed as a 38-55 on steroids.
Need some parts to complete the action. When I got it all it was,was the action and a barrel attached.So basically need complete trigger setup,saftey,triggerguard, and a few other parts. Then I do the prep work prior to blueing. Probably bead blast and blue. Frank

Wayne York cut my 38-55 barrel (375) with an 18" twist.
It works just fine with the Lyman 378674 330 grain bullet.

Frank, just go with what you got and you'll be just fine. Don't worry about others trying to convince you of a slower twist. Old habits in firearms die hard. One of them is the slow twist crap. The 375 Winchesters shoots just wonder and very accurate with it's 12 twist with either cast or jacketed. Enjoy your project.

OK,will go with the 1x12 twist barrel. Got that barrel plus about 6 others as full sized blanks. The barrel in question was partially turned down but savable.have to turn off about 1/2" to get to good rifling. never threaded a barrel before so I have scrap aluminum that I can practise on. Thanks again,Frank

Good deal Frank, let us know how it turns out and shoots. Be careful cutting those threads.