I just did a search for FEAR nailgun on youtube.
http://uk.youtube.com/results?search_query=FEAR+nailgun
You guys are right. That weapon is awesome. Mine would be multi-shot though so at least it'd be a little different.
I think it could work something like.. when you land nails into an enemy, it pushes them back a little, but if they're close to a wall, it ends up pinning them to it. And then after you've killed them, you can see them nailed to the wall and all your nails sticking out of them (maybe just use existing animation frames that look the most appropriate for being nailed to the wall). That'd be nice.