and actually, i'm pretty sure those guns in co-op are only available in multiplayer if you unlock them in co-op. in other words. you can't use them online any other way. i'm not positive, but pretty sure.
luckily I have someone to play with. co-op is a *** though. i played the first 3 on normal, and that third mission was difficult even on normal. there was a section after we got the hostage, where enemies were spawning behind where we were - RIGHT IN FRONT
OF MY EYES. ... that doesn't really make sense, but i think you get it. it was unbelievable. almost lost the mission because of that bug.