I started shooting just as plastic one piece wads were starting to be the norm. I am the "numbah one cheap Charlie", so I have shot hundreds of factory and handloaded card wads. All the problems I remember reading about with "shot lumps" were black powder loads with very soft shot. Could lead oxidization cause shot to stick together in a lump? I suppose it's possible. I don't use card OP wads by themselves much. I use old Alcan PGS or Air-Wedge or BPI's obturators with a card over them (mostly to prevent powder migration). A lot of older guns were choked for card wads and are tight with plastic one piece wads. Card loads without shot cups will let an extra tight choke pattern to it's original boring. However, loading card and fillers is a big hassle and I don't load a lot of them for target practice.
