The original .410 muskets used a straight walled .303 based cartridge that was more or less a .303 that hadn't been necked down. The chambers are too short to accept a standard .410 shotshell, but some American importers had a number of them reamed to accept them. It's an easy conversion, but, to some collector purists, makes the gun not original. The guns themselves are not particulary scarce.
They are fun to shoot, though; Recoil is almost nil.
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