[Garpman]

FTF = Failure to Fire
FTF a = Face to Face (used a lot on gun forums to conduct sales)
WTB = Want to buy (again for marketplaces)
WTS = want to sell " "
WTT = want to trade " "

FTE = Failure to eject/extract

FFL = Federal Firearms License

NFA = National Firearms Act

While a dictionary is a good idea, once you spend more time around gun forums, or talking about guns with people, it will soon be made clear.

As for NDs and ADs, someone put it real nicely on another forum:

AD = malfunction of firearm or cartridge
ND = malfunction of you