Just out of curiosity, why do you need 7/8-16 Threads? 7/8 ucn is 9, 7/16 nf is 14, and 7/8 unfe is 20. So 7/16-16 would be a special, are you sure that you have a barrel threaded for a special thread? It would seem at least quite unusual, if not downright unlikely.
An M14-1.0 would seem close to what you are asking about. Just askin'.