This is actually a really hard question. There are movies with egregiously stupid bits of gun dialogue (DIE HARD II's infamous "porcelain Glock") that have decently entertaining gunfights. There are movies where the gunfights are utterly unrealistic, but still entertaining as all hell (most gun-fu films, and EQUILIBRIUM -- hey, *I* liked it, dagnabbit). And there are movies where the gunfights are poorly staged, but not all that offensive in terms of accuracy. So this really is a very tough issue to think about. Good one, Ogre.
Hum. Well, we're not talking bad gun movies, but we're talking bad gun *fights.* So, that said, I concur with folks who say the major contender for the worst gunfight in film has to be the final assault in COMMANDO. It's not just unrealistic, it's dramatically unsatisfying. 1) People shoot at Arnold, and miss. 2) He shoots at people, from the hip, full auto, killing crowds with every shot. 3) Repeat for five minutes. In my stupid action movies, I want to see the hero think creatively or try something new. Watching that scene in COMMANDO is like watching your roommate play DOOM.
And the thing that gets me is, I'm trying to think of another gunfight that's that bad, and I just can't.