you'd probably need something very small, that breaks into other pieces that resemble normal stuff like make up holders/pens etc, that are also shielded from chemical sniffers and made of stuff that doesnt alert MAD detectors.
very high social skills to bluff it through security when they undoubtedly have a look at it.
someone on the inside who can get ammunition into a high security area and get a means of getting it to you inside the terminal
total idiocy to then use the weapon on a small, flying, crowded, aliminium tube that you are stuck on with a whole load of witnesses to your crime, until it is landed in an airport full of police wanting to arrest you.
probably the dumbest plan ever, just tail the dude on his way to the airport and kill him in his car/the car park/on the way to the terminal/in the terminal, just not on the bloody plane lol.