Re: Flintlock Shotgun
double flint shotguns are extremely expensive. A single barrel flint smoothbore can be quite reasonable. Options run from a repro brown bess or trade gun to buying an old rusted out renegade barrel and having Bobby Hoyt ream it smooth for you. another alternative is buying a smooth rifle in a large bore Although either of the last two options would have heavy barrels and are not easy to swing and fire at flying game.
Used smoothbore flint lock trade guns start at three or four hundred dollars and go up fast.
Keep an eye on Track of the Wolf and a few other places for new and used guns.
Years ago, Thompson center sold a smooth bore 56 caliber gun called the New Englander. Not sure if or how many flint models of it were sold. Parts (barrel and stocks) for New Englanders are frequently listed on the on-line auctions.
If you already have a Lyman or TC gun in flint, buying a compatible rusted bore barrel for 50-70 dollars and having it reamed out for (IIRC) about $80 to $100 will get you shooting shot, but isn't the best option.
There seems to be a demand for a reasonable priced smooth bore in flint, but no suppliers
You could also check M Sikligar, Veteran Arms and Middlesex Village Trading for their smooth bores. Some Indian guns are fair quality and some use steel tubing for barrels which is a subject of debate on many black powder sites right now. Years ago, Navy Arms sold some Indian made muzzleloading doubles and they performed as designed.
I just ordered an Indian made 20 ga pistol from Veteran Arms, Haven't received it yet to be able to describe quality. I have ordered match locks (just the locks) directly from M. Sikligar (in business for about 50 years) and they were mirror polished and worked smooth as glass out of the box.