Hit Points are weird. This is hardly news, I know. Entire volumes have been dedicated to providing some sort of logical justification for them as an abstraction of health, luck, mobility and anything else that comes along. They might literally represent progressive injuries, but more often they represent you slowly getting tired and generally roughed up until things get really bad.
We've trained ourselves to be comfortable with this in a lot of games, but it always hits a little resistance when we add in guns. We have a harder time abstracting away the effect of guns for a number of reasons, the biggest being that it's very hard for us to reconcile the idea that a hit with a gun can be casually shrugged off. That cuts to the core of the "health" component of hit points and makes it difficult to sustain. We can imagine the occasional grazing hit, but those quickly strain credulity. On the other hand, using hit point purely as luck or agility to produce retroactive misses is pretty dull, and especially frustrating when players have guns to tell them their hit is a miss, but it's really a hit, you see.
One solution to this (beyond making guns stupidly high-damage weapons) is to decouple the scariness of guns from HP damage and instead reflect it with effects or conditions, and that definitely works well, especially if you want to introduce guns into an existing game.
But if you want to do guns from scratch, as in to just do a simple firefight with 4e, a more drastic departure might be in order, one that's been on my mind.
The idea on my mind is to turn Hit Points into a more generic pool, perhaps just called luck, reduce them, and make them a component of _defense_ rather than damage. The trick for this is the addition of Defense actions, At-Will interrupt actions available to all characters to represent things like diving out of the way or making a block. These actions do something descriptive, and also raise the character's defense by the number of "Hit Points" spent. In the fiction, the _action_ creates a miss.
Mathwise, the result is very similar - HP slowly ablate from hits - but descriptively it creates a much clearer sense of when a hit connects and does damage.
Speaking of damage, this would of course call for a slightly different damage system, some sort of injury model. Perhaps an injury threshold - if an attack that hits rolls damage in excess of the threshold, you're out. If not, reduce threshold by the # of dice rolled, representing an issue.
It's still a kind of loose idea in my head, but some part of me has been wanting to take the quick play of Gamma World and drop it into something closer to Feng Shui (except, perhaps, with Planescape cosmology). This is a bit too much of a departure for straight D&D, but for a departure in the same style as Gamma World, it might be reasonable.
I may have to try out a firefight or two soon to see how this fleshes out.
1 - And, in fact, a cinematic shorthand for things being out of the ordinary is a target that shrugs off bullets.
2 - Which would also demand HP not be tied to Constitution.
3 - Including @Wm_Bounty's brutal "Shot" condition - make a death save at +4 each round, Heal check DC15 to end.
4 - Maybe fixed values. Maybe some # of dice. Need to play test what's fastest.
5 - For straight D&D, @gamefiend pointed out the the Star Wars Damage Threshold (You have HP, but a single attack doing X or more damage has certain effects) probably works well.