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A friend of mine have a H&H royal from 1915 ish in 500/465. He is considering to have an new set of barrels made to it in 500/416 as he wish to have a 416 cal to use for hunting.

But im consirned about pressure and if it would bee any danger to fail Proof?

whats your thoughts on this matter gents??

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Why not ask Holland & Holland and the Proofhouse? Quite logic, or? I'm quite confident they will let your friend know if there are any "issues" on this matter