Crow 1, you are fortunate to have obtained a rifle from W.M. Large, he has an excellent reputation for quality.
If you go to page 11 in this forum, the first reply is a photo of my 50# RB rifle with FM.
It is a slow twist 1/72. I like you have just obtained the rifle this winter and have not shot it yet, we have had a long cold winter here in New England, but things are looking up.
I'd say there is a very slim chance of this being a slug gun. Many of the modern era ( 1930's, 40's, and 50's) smith's put false muzzle on their round ball bench guns. Spence and Wolf to name two. The twist is likely to be 1 in 72 so you can beef up the load. 165 to 185 grains of 2ff, but check that drum and nipple before you shoot it. The patch ball combination mentioned earlier sounds about right. Have fun. Jim