Unless you have substantial pitting, Gamebore should work well. I shoot a Modell Nr. 2 in 16 bore quite often & the chambers have been lengthened by some astute fella to 70mm. On occasion I might shoot a few 70mm 16 bores but recoil is going to be a factor in a light hammergun. I was just thinking of ordering either some 12 bore Gamebore Pure Gold(78xx PSI) or more than likely some Gamebore Traditional Paper(65xx PSI)_(my preference) from Firearms Service, a Kent / Gambore dealer in Kentucky. But you are much closer to the Gamebore source & might reside adjacent to it?

Cheers,

Raimey
rse