The Thompson in NA is hardly underrated but from personal experience I'd say the one with the 20-round clip is much, much easier to use and better to aim than the 50-round drum, and that if you are using the 50-round drum, putting it in single-shot improves your killing power greatly in any situation that doesn't involve storming a house or a bunker. In single-shot I treat the Thompson a bit like the M1 Carbine, three quick shots and not only does it improve aiming dramatically but it makes a 50-round drum last as long as it feels like it should for once.