Charls,
Gibbs made a few 'Field Patent' rifles in 0.461" No.2 Gibbs calibre, and possibly in other calibers, most of which are likely to be obsolete today. It may be possible to trace the original calibre if you have the Serial No. In fact the calibre may be stamped in the action somewhere, if you look hard enough. If you can ascertain the serial no., it may be possible to trace the original calibre your action was chambered for. If you cannot or are unwilling to have it re-barrelled it to the original calibre, then select a cartridge of approximately the same calibre that is available from ammunition manufacturers today. 45/70 does seem a likely calibre for todays usage although it is highly unlikely the Gibbs Action was ever chambered for it.
{I have little doubt that my Internet friend Gert Claes (MartiniBelgium) will chime in here}.
If you think it has no value, think again. British Single Shot Rifle Actions are much sort after. I'd love to make you an offer for the action alone, but I don't think I could afford it. lol.
Restoring this action to it's original condition with the correct barrel contour and the right sights and stock shape would increase it's value considerably. If you cannot be bothered to go along this route, I am willing to take it off you for a reasonable sum, heck I'll even pay the postage. PMSL.
Harry.
