Ok, i tried to make a clean install, update to 1.05 and then install unbundled 3.0 weapon pack, and started the campaign coop mission10 mission(also tried mission1 with no luck). We did also try to host on the other computer and see if the problem was still there, and yes, clients had the MR-C instead of MK14 which the host has.
I noticed a new thing though, i checked the weapon after the player died and when i went to look at the weapon it said "MR-C old" maybe it'll help you a little...