I suspect it is a shoulder mounted gun, muzzles measure 1.820 across both barrels, .750 on the inside and .910 outside at the muzzle.
At the breach, the barrels measure 2.500 across both barrels outside and each barrel measures 1.320 outside.
If this gun has a .750" choke, it most certainly is not a 4 gauge or an 8 gauge either. Sounds like it may be a heavy 10 gauge waterfowl gun with rather long and heavy barrels... not a punt gun. Since it is a percussion gun, and may have been built before choke boring was common, it could be an 11 gauge.
http://www.hallowellco.com/bore_size_chart.htm
