There's so many variables involved with shooting BP guns anyway, that it all boils down to it being more of an art than a science.
How perfectly consistent is any round ball, or powder charge, or cleanliness of the barrel from shot to shot, or ramming pressure?
If we start being too picyune about all of the details, then we begin to forget what shooting BP guns is all about, i.e. - the simple art of shooting, and not
so much the science that it involves.
We can always turn BP shooting into an as sophisticated or crude activity as we choose to, but do we really want to go that route?
And then there's the question of which style of gun can predictably win any given accuracy competition. I'm not convinced that a fixed breech is inherantly more accurate - someone will just have to go ahead & prove it to me every time each individual gun is fired.
I highly doubt that science even exists in the black powder shooting universe. After all, everyone should realize that there's magical spirits that guide the flight of every ball and bullet fired which totally defy all known logic!