When you say Piraka Guns, do you mean the Zamor launchers, or the other things? Because I managed to use the Zamor launcher as part of something's torso. Dunno if I'll enter, though, I'm not so good at MOCing.
..While I'm on the subject, would the upper torso of Mask Of Light era Matoran count? I made a Toa of Fire that has those as part of its arms.
How about Vahki heads? Not the braincases, the actual head part, used as something that's not a head.
I also remember putting together something with Rakski spines for upper arms years back (and a Lego Hockey helmet for a stomach), but it looked pretty silly.