For everyone trying to do the achievements for Onslaught that require you to use "Smart Ammo" here is a great idea that H3LL She-Devil showed me and I wanted to pass along to you too. That allows you to restock the "Special Ammo" without having to get 2 kills in 2 seconds. Best place to farm is Russian Embassy . Which gives you enough ammo boxes to keep refilling your "Special Ammo" and not have to earn it.
https://youtu.be/hPecm59vMmQ

