I bought a NewEnglander .50 years ago. I picked up a 12ga barrel for it and liked it. I added a camo stock a couple years later, then decided to buy another lock and have 2 complete guns.
The NewEnglander is a fine gun. The 12ga will handle round ball loads (with an open choke), but I've never been able to get much accuracy with them. (My .62 smoothbore flintlock did much better.)
With more experimentation with loads I'm sure I could improve accuracy, but I just never took the time since the .50 cal handles those hunting situations.
I did buy a full choke tube (and an extra full .660) for working up a turkey load for the 12ga. It does very well with both tubes, but I usually keep shots under 30 yards just to be sure they're quickly fatal. No quick second shot makes you a bit more selective in the shots you take.