I think I've come to the conclusion that Mistborn could, in fact deflect bullets. In fact, I wonder if it would even be that hard.
For one, the coins they shoot must be approaching bullet speeds anyway. These are dull edged coins, not sharpened arrowheads, yet they tear through flesh with a lot more force then an arrow. Don't most of the coins go THROUGH the persons hit by them?
Also, after just rereading that passage when Vin opens the big vault door by pulling on the metal in it and the metal plate behind it, I'm even more inclined to believe that they can generate the force needed.
The only question would be how long could they sustain a 'push'? You don't know when the trigger is going to be pulled, and once its pulled its too late. But then again, seems that you have the same problem with coinshots, so I think Yes, a Mistborn can deflect bullets.
I believe a new pair of metals slow down time or speeds it up....Cadmium and I think it was cerrobend, but it could have been changed.Pushing can be sustained as long as the blue line is there.