How to get a bit more early Minigun ammo.

In an area with someone who uses a minigun plus other raiders or whatnot, kill the one with the minigun but not the surrounding raiders. One of them will pick up the minigun. Then you can kill that one and so on. Each new person who grabs it magically spawns 5mm ammo in their inventory. Just now I killed Ack-Ack with a grenade and a nearby raider grabbed it and grew 500 rounds of 5mm in his inventory while leaving Ack-Ack's ammo on him. The same trick can be used to get plasma or other scarce ammo, though I've never tried it with a missile launcher or Fat Man.