A friend and myself found a way to very quickly unlock the guns in co-op - approximately one every 20 minutes. How?
Go to the final mission, "The Eleventh Hour" and put it on Easy. Simply run through all the enemies (shoot a few on the way so you don't get knifed) until you get to the part where you're supposed to defuse the bomb. When you get there, press any random button so the bomb explodes and you fail the mission. You will get roughly 4500-5000 score every time you do this, and it takes just under a minute every time. Since the guns unlock once every 60k score or so, it'll take ABOUT 15 minutes to get each gun.