I read an article in the 1930's American Rifleman. It was about long barreled black powder guns used at target practice in Tennessee. They look very similar to the rifles in the "chunk guns". It is my recollection that 50 caliber was big for the things, but caliber did not matter to scoring. Scoring was based on string measurement. You provided a ball, one that you would have shot in your rifle, and the center of that ball was where the string measurement was based. Starting with the X you made on a piece of wood, you shot at the X, and the distance from the center of the X to the center of the hole was made with a string, using the ball to judge exactly the location of the center. I think they added the distance to the center of the next shot, but it could have been the string distance from the center of the X. (could use some help here) The shortest string length of the for the group, won the match, and the turkey, if that was the prize. I think they were firing five shot groups, could have been more. Distances were less than 50 yards. You had a group of people, a group of judges who had to measure, so having everyone walk up and down a 200 yard line each shot, would not have worked.
Based on the stock, that rifle looks post WW2, the historic rifles I have seen did not have that buttstock or cheekpiece. The firing mechanism could have been purchased in the 1960's, underhammer replica mechanisms were being sold then. I don't know about the barrel, if it is soft iron, it could be much older. Backwoods gunsmiths made their barrels from iron and those flats were cut with a draw knife and finished with a file. When the barrel was shot out, it was bored for a larger caliber.