First of all, welcome to the Muzzleloading Forum! I didn't notice at first that you are a newer member. We are glad to have you.
I would agree with
@Bucky in that I count seven "sides" in the bore. I believe these would actually be the lands between seven grooves in the barrel. Sometimes in looking in the muzzles of old rifles, the lands and grooves are not distinct, and look like flats, like yours. So, judging from the muzzle, I think your firearm is probably rifled. Seven grooves would be a very common number for older rifles. I am surprised that the bore is as large as described. If it really is 0.50", that would be fifty caliber, which was considered adequate for pretty big game, back in the day. I was thinking this might be some sort of gallery rifle, but would have expected a much smaller bore. With that weight of barrel and that bore size, I'm not sure what to make of it.
Length of pull or LOP is essentially the length of the buttstock. Smaller people need a smaller gun in order to shoot comfortably. Measuring is easy. Lay the rifle on a bench or table, then lay a yardstick on the butt stock. Align one end of the yardstick even with the deepest part of the curve in the butt plate, and measure from there to the center of the front trigger. Most old rifles will measure between 13" and 14". A normal sized modern-day male will probably want an LOP around 14". Smaller fellows, like myself, shorter than about 5'8" or so, and adults who bundle up in heavy winter clothes, will want a shorter LOP of around 13" or 13-1/2". Rifles for children may go as short as 11" or 12".
If you don't mind, I would ask that you use calipers and measure the diameter of the barrel at the breech, and measure again near the muzzle. This will tell us if the barrel is tapered. It appears to be tapered in the photos, but that may just be an illusion from the perspective.
One other thing... The sixth photo in your second post
appears to show some exposed threads, where the powder drum or bolster screws into the side of the barrel, maybe due to corrosion. Yikes!!! I would not try to shoot this gun until it can be thoroughly examined by someone knowledgeable about muzzle-loading firearms.
Again, thank you for posting, and for the excellent photography. We'll see what others have to say about this unique and interesting rifle.
Notchy Bob