How does that seem odd? Its the future, theres this crazy bad ass shield technology the military created that gets "overloaded" by high velocity impact, electronic current, emp's, explosions, etc.
The sheild is designed to cover a certain area, like the human body for instance. Once the sheild becomes "overloaded" it shuts down to build up enough energy to re-activate.
Do you realize how much energy it takes to make multiple rounds of big ass bullets just bounce right off you??? Neither do I, but I'm sure its a $h*t load.
What wouldn't make sense is a sheild weaker in some areas and not others. Sheilds should not have weak spots. Thats the whole point of a sheild. Imagine if Link's sheild had a weak spot!? Ganondorf would still be running Hyrule!
Think of this futuristic shield as a forcfield thats invisible to the naked eye. You wont even know its there unless you hit it hard enough with something. If you tried touching it, you wouldn't notice anything was there, you wouldn't even feel it. If you punched it however, (assuming your punch is strong enough) you'd be in for quite a surprise after the shield breaks your hand.
So, pop that sheild, then go for head shots, or if you're close enough, melee, cheif packs a mean punch.