I've got a project coming up for a good customer of mine. He wants a 505 Gibbs Magnum, built on an Enfield action. Instead of doing the 505 bore, we are going to use a 510 bore (more bullets available in this caliber). Anyway, I'd like some opinions on twist rate. He's going to shoot a range of bullets from 450 gr. to 600 gr.

I don't have a lot of experience with calibers of this size, so I'm having trouble deciding for certain on a twist rate. I'm leaning towards a 1-14 or 1-16 right now.

Let me know what you guys think.

Thanks.

Might ask the guys in Big Bore section. No clue from me!

Jeff - wouldn't Shilen or Douglas be able to help you out?

You need some of the bullets he's going to use. Get the LONGEST (not necessarily the heaviest). Meaure the length of the longest bullet type, then use the Greenhill formula to calculate the correct twist. This formula uses calibers as a unit of length for both twist and bullets. It says:



Required rifling twist (in calibers) = 150/length of the bullet (in calibers)!