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 1.2KG / m^3 which means the bullet is encountering around 0.058 grams of air in its flight path. 

Via E=MC^2, 0.058 grams 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 annihilating in a terrific mushroom cloud. Maybe next time, we should save the antimatter weapons for Space!