There is no 'industry standard' with respect to anything about percussion caps or nipples. Nipples threads do generally, but not always, conform to one of the common thread specifications, but there are at least 10 different ones in common use (meaning you can find them in a catalog):
The catalogs will sometimes specify 'rifle', meaning a fairly long cone, or 'pistol' or 'revolver', meaning a shorter cone, but that's all.