Minigun!
6th floor of the air control tower at Fort Zancudo. All credit goes to xXMeLikeBigBoomsX for the tip.
This is how I did it:
They way I got it there was the "Target Practice" mission glitch, on my 2nd play through.
This is a cheat-free glitch in the game that (still) works as of 1-8-2014 on XB360.
Before you start the Trevor mission it helps to have bought the SS Hangar, as we will be looting Fort Zancudo.
Start the mission. Instead of following Cletus up the stairs, set destination point to the FZ entrance north.
Call a Taxi or drive there, Taxi is faster. Enter the base and find the tower. Climb the stairs to the 6th floor.
The weapon is right there on the floor. Re-enter as many times as desired, for 1,000 rounds each spawn.
(I stopped at three times = 3,000 rounds, but you may want to try for more yourself.)
Then grab the Lazer and fly it to your hangar and park it. This is the equivalent of the Hydra in GTA San Andreas, with full weaponry, and also works great in the knife flight missions. Taxi back to the Fort and grab the Buzzard. Fly it back to your hangar and park it in the helicopter spot. Taxi or drive back to Cletus to complete the mission.
After mission completion saved you will have the minigun, Lazer, and Buzzard.
NOTE: This is a no-kill humans mission so you may or may not have difficulty stealing the Rhino Tank,
much less a facility to store such a large land vehicle. I played with it but did not store it as obvious notes.