You would need a sizable drone to pull this off... something at least human size and/or weight.
Bullets impact with a LOT of force, and a small drone would just get bounced about from the impacts and not provide any real benefit.
That said, a drone with that much mass also IS a kind of shield for the rigger just by its size. The rigger could just hide behind the drone for the +2-4 defense bonus....
I don't think you will find anything in the rules to help you figure this one out... as its kinda counter to the typical 'get out of the way is best' mentality when it cones to firearms :p
Also, not really seeing how armor on something/someone else helps ME. Sure, it makes the guy/thing with the shield tougher... but that has nothing to do with me or my protection... at best, he provides me with cover from incoming fire... but that doesn't help me any once the bullet gets past him/it (like armor would)