I really like this idea! I can imagine being in a tight spot and having to resort to a firearm to clear a path to safety to only have it jam and require a cleaning kit with even more zombies bearing down on you from the first few shots. Maybe instead of a cleaning skill have a firearm maintenance skill that affects how fast you can clear a jam and the efficiency of firearm repairs. Also having the frequency of jams raised as the weapon degrades would force you to keep your gun properly maintained if you may need to rely on it to save your life.