This is a difficult challenge. You have 30 seconds to shoot two sets of moving concentric targets and one stationary target with M16A2 Rifle, 20 rounds of ammo. The rifle is set to three round burst, so trigger control is the key, you only have 20rds for seven targets so conserve your ammo. Out of ammo = game over.
Try to knock down the front set of concentric targets first, after you tried that, get the stationary target (easy), then the 2nd set of concentric targets will appear, get them. Repeat until you get them all.
The concentric targets are small and moving fast, and one set of them can only be shot at in 1/4 of the time, not easy!
Good luck, remember trigger control, only fire when you think you can hit.