A PIECE is a gun, used by the Scoundrel class. The terms 'Piece', 'P.I.E.C.E.', and PIECE are used interchangeably throughout Starmourn, as it's become somewhat of a colloquial term for any kind of blaster or gun, rather than specifically the iconic multi-ammo Prion Industries Electroswitching Compact Eradicator - kinda like how lasers don't get called L.A.S.E.R.s and radars don't get called R.A.D.A.R.s all the time.
While Prion Industries invented the PIECE, they've been out of business for decades. The name, however, has stuck.