It is REALLY hard, the enemy has to literally have ALMOST captured the flag and then you shoot him. A good way is to see the flag go white, count to 3 if there is only ONE person at the flag, and then SHOOT!!!!!
The correct answer is to let the enemy capture four flags. As they are capturing the last one (the fifth flag), take out whoever is taking the last flag and capture it to keep the game going and it should pop.