MSMC. I wouldn't say I sucked with it, but after hearing good things I was expecting it to be "PDW good". It's one of the few guns that has regularly caused me rage by not killing after 6 or 7 hitmarkers. I tried rapid fire on it for a few games and it felt even worse for some reason.
Onto the Chicom now, and that feels a bit guff too, but I don't think that's a highly regarded gun by most people anyway. It feels powerful enough, and the burst fire is fine, but I just don't like the way it handles for some reason.