First off I apologize for the mic feed back about a minute in. Don't worry it doesn't last long.
This achievement is obtained in the level Firestarter Day 1.
Once you find where the weapons are located you must then bag all 10 of them. Do not destroy any of them, you want to take every one to the van and steel them. This will require a lot of back and forth carrying and tossing of bags since there are 4 players and 10 bags. The guy on the radio lets you know how many you have. When you get all ten he says "Ok, you guys totally emptied that hangar". Once he says this hop in the van and get out of there.