Yeah, to me it looks like they tried to "weather" all the guns. Doesn't make sense though, take a look at the MK 16 now... Most of that gun is made of polymer that doesn't "wear" to a different color. Only the upper receiver of the MK 16 is made of metal.
I sorta like the new graphics makes the game look a little cleaner. It's not just the guns its on alot of textures. Not sure if they did that one purpose or maybe a side effect of it seemingly like they removed a lil more bloom from the game.