Let's say the bullet is a standard .308 Winchester round with a diameter of 7.85mm which means the bullet is encountering: 

$\pi * 3.925^2 * 1000 = 48,400$ cubic mm of air.

The density of air at 20 degrees C is $1.2 KG / m^3$ which means the bullet is encountering around $0.058$ grams of air per meter of its flight path. 

Via $E=MC^2$, 0.058 grams of matter is equal to **5220000000000000** Joules of Energy which is more than the **3000** or so Joules that was imparted to the bullet on launch.  At this rate the bullet won't even make it 1 meter (or 1mm) away from the rifle before the explosion on its tip bring it to a sudden halt and then annihilating itself a terrific mushroom cloud. Maybe next time, we should save the antimatter weapons for Space!