Earlier quoted context omitted.
Bullets carry significant externalities. How many of today's regional conflicts can be traced back to mass atrocities? Birth control doesn't create survivors perpetuating rage based on past martyrs. Even if the immediate efficacy approaches your suggested value. In truth, it's almost certainly closer to 100s.
Wars lead to even greater population reduction. However, once one baby sneaks past the birth control, then you have to start all over again with that new person to teach them to not create yet more babies. War and vengeance is a viral population reduction method.
In the 20th century, the Spanish Influenza Pandemic killed more people than WWI. And WWII. Combined.