I haven't used riot shields yet in MW3, but I have come across a few people who have. In my experience, 9/10 times I can take them out while shooting straight at the shield, even without them having taken prior damage, as well as also getting easy knife kills against shield users. To me, this makes it seem underpowered. It should take more than a few bursts direct to the shield, or 1 knife shot to kill a shield user.