Here are a few common on gun forums
FTF:
Failure To Feed
Failure To Fire
Face To Face - for sales
FTE = Failure To Extract or Eject
OP = Original Poster or Post
BTT = Back To Top
BG = Bad Guy
MOA = Minute Of Angle
AR's have a whole sub set unto themselves. BCG, FSB, BUIS, FA . . . . .