In MI, a BP rifle with no cap or primer is considered unloaded by the Department of Natural Resources who manage hunting. So, you can transport it in your trunk with powder and ball in place, just no primer. And, you can walk with it in that condition even before the hunting period starts for the day. Once you put the cap on or insert the primer, it's loaded, and you're considered to be "hunting" so it better be after sunrise (as described in published time charts).
At a DNR range, however, a BP rifle with powder, ball, and no cap or primer is considered to be loaded. So, nobody goes downrange until that gun is fired.
As far as the argument goes, once a BP rifle has powder and ball in it, it takes only a few seconds to put a cap on in and shoot an aimed shot. Not worth an argument.