Chunkguns generally have flatter butt plates. Since they are shot from a prone position, I think the deep curve of that butt plate wouldn't fit the shoulder well when the shooter is lying down.
So, I would call that a bench rest target rifle rather than a chunkgun. Very nice-looking rifle, by the way. However, of course, chunkgun matches can be shot with just about anything so long as it loads from the muzzle, uses real black powder, round balls, and has fixed iron sights. They can be either percussion or flintlock.
My chunkgun weighs in at an even 16 pounds. They can be much heavier.
Here is mine. Bad angle on the photo - built for me, left-handed, barrel is 46", 1 1/4" across the flats, .50, double-set trigger, back-action lock. The protrusion near the end of the forearm is called a "cant block". Mine is built-in, but they can also be detachable. Mine also has two brass "pins" on the bottom. The purpose of a cant block is to rest this on the "chunk" when you are in the prone shooting position. Cant blocks are not necessary, but they are a nice touch. Many "chunks" are now homemade, but originally, they were simply a section of a log, some still use a short log (firewood) section when competing in chunkgun matches.
