Casper

The sizes you give do seem different to a usual .410 but do remember that as snap caps are hand made or at least run off in limited production numbers and are not subject to the same dimensional tolerancing as loaded ammunition, then sizes between different makers may vary and the caps will always be smaller than chamber size for obvious reasons.

That said if there appears lots of taper on the side walls then I would venture to suggest some form of rifle snap cap as has been said already. Most shotgun snap caps are parallel give or take a few thousands of an inch of taper. I do not have any case sizes before me at present to suggest what other chamber(s) they may fit.

If you wish for genuine W-R snap caps whith named headstamp you may wish to try Westley Richards in Birmingham, although they don't show them on their website they may well have a set in stock or have some run up for you. If you do have some made ask if they can be made by Mike Marsh. They know who Mike is as he makes most of their best quality case tools as indeed he does for Purdey, Boss and H&H and a number of thye other famous names still in business.

Mike makes snap caps from a variety of materials including Nickel Silver, brass/ebony; brass/buffalo horn; and has also used ivory. The cost will be higher than run of the mill snap caps but then again the quality will be higher. If they are to go alongside your gun in a best quality oak and leather French fitted case then it would be worth the investment.

Jonathan