Wouldn't be hard for a soldier to disassemble a rifle and bring it home in his kit when on leave. There were thousands whose owners no longer required them lying about after the battles, never to be accounted for unless collected by a salvage party.
A farmer always had a use for a rifle and why not get one for free? Or a man from a rural area who fancied doing a bit of poaching after the war. And if not that, at least a good way to supplement the King's shillings. We forget how little the average man had in those days.