1. play the first dreadnaught mission (doesn't matter what difficulty)
2. get to the end where you have to destroy the tank
3. your ghost spawns heavy ammo for you
4. free full heavy ammo for you to do whatever you want with
5. if you do it on normal mode and go to orbit after getting heavy ammo, it gives you a checkpoint to go back to so you can just go to the hull breach and pick up the heavy ammo after it spawns without having to deal with the "shutting down the weapon" part of the mission
edit: whatever. I guess everyone who plays destiny except for like two people think this is a stupid idea/method. I'd say that without using a synth that this is the fastest way to get full heavy ammo, unless you're lucky as -blam!- and get like 4 enemies to drop it in a row to fill your heavy. For me, synths are for in a raid type situation where you need ammo right now and it's not dropping, and if I want to fill my heavy instantly doing whatever random stuff like the daily and bounties or whatever, I can just go into the mission, walk over to the hull breach and wait a minute and get free full heavy.
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It sucks we have to do this. If only bungie would design a consumable that you could use to replenish all your heavy ammo.