Well here's what Naughty Dog had to say about it.
A look at gun mechanics in single-player and multiplayer
It definitely feels very awkward at first and I had to play around with the sensitivity longer than I would have liked but my my biggest gripe regarding the gunplay is how enemies often don't react to being shot and will keep firing at you. Also they love flanking you from behind, often coming from off of the map so you had no idea they were even there.
I've also found that it's not as easy to pick ammo. Often I will go to roll over it to pick it up and miss it. Also you almost have to be standing still to reload.