(HULK) said:
The show runners comments + the fact that the gun ran out of ammo 17 seconds before the dude shot himself in the head makes me think we saw a hallucination of some sort.
To layer onto it, what if the shot came from someone else and that is why the splatter hit Glenn in the face like it did. Way too much splatter for a gunshot to the head like that, it wasn't a 44 magnum afterall. So the mystery shooter comes in and wipes out all the walkers and voila...Glenn lives.
That's a lot of walkers to get rid of who are all on top of Glenn.
If Glenn's alive, I'm starting to think this is more about him killing Nick and somehow becoming seriously messed up as a result. If Kirkman's comments at Comic-Con can be trusted developing Glenn's character in a completely new way as a result of him doing something he'd never done before (something Gimple references in the article I posted) would line up as something cool to see. But ultimately, it all resides with a believable explanation for how he didn't die based on what we saw. That's on Gimple.