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I've just bought a takedown Greener Martini 12 bore rifle marketed by London Guns (UK) for a spot of fun at the Big Game Rifle Club. I hope to eventually make my own reloads, but to get started, can commercial 12 gauge slug cartridges (non-rifled slugs) be used safely in a 12 bore rifle? Are any other commercial cartridges available?

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I doubt a commercial slug will shoot well in a fully rifled 12 bore from Greener. I think the slugs could be smaller...but you need to find out the twistratio of your rifle first. If the twistratio is only one quarter spin of the barrel like 1:72, you have a gun that can only shoot roundballs only.
If the rifling spins more than a half the barrel length like 1:50, it can shoot projectiles too.
You also need to have a chambercast made, so you can determine ratio between the bullet dia and case walls aswell.
There is no commercial ammo made for 12 bore rifle. I know H&H can supply you with paradox ammo upon request.
Congratulation with your purchase