There are also harsh accuracy penalties for firing while in the air, but these don't matter if somebody is literally pointing a gun barrel into your eye socket (it's exceptionally hard to miss at that distance). Generally there's nothing that can be done to combat that sort of thing at all though, short of actually disabling a player's gun. No amount of random accuracy or screen kick is going to stop the first bullet from a player's gun from getting a headshot from three feet away without being so extreme that it hamstrings other aspects of the gameplay. Aerial accuracy is quite bad however, so if you're any distance away you should actually have more of an advantage than them at least until the other player lands.
"What wonderful things are these chemicals, spores, and strains! They give life, restore it, prolong it, torture it, and take it all in unison. They predate us and we are spawned of them, and yet such things can unmake us with curious ease. If such things are not god, then I have yet to know anything more deserving of the title!"
-Dr. Karov Icari, taken from Biopreparat development notes, 1993.