How much did he move? The classical evade where he barely move, or did he pull an Albert Wesker? Evading a bullet is far from near being as fast as it. A Formula One car drive in over 300 kilometers per hour at some occasions, but by seeing its line of travel and discover it in time, even a can evade it. Same goes with evading a bullet. Sight it in time and you don't have to be even half as fast.
All it prove is that Zack has good reflexes and good speed. Not necessarily the speed of a bullet. If he is shown moving at that speed, I will give up. The speed of a simple handgun bullet is nearly 1000 miles per hour. Equivalent to 1400 feet per second. And that's not even a powerful handgun.