If you change out armor that gives you a bonus to heavy ammo before you respawn you will not lose ammo. Unless you had ammo over your cap. You will lose that.
This is the heavy ammo glitch.
It takes some getting used to, but it's really important that your primary ammo is 0 and doesn't regenerate before all those Acolytes are killed.
This is what your screen will look like when you do it right. On average those boxes are worth 4 rockets.
I think you need a heavy with surplus to get the most out of this glitch. It's also sort of annoying to do, especially if not everyone on your team is on the same page and people keep rushing to summon Crota or you're wiping all the time. Even using Universal Remote or No Land Beyond it's still really hard to empty your ammo before the gate comes down.
Is it better than using heavy synths, which are stupidly expensive and require glimmer grinding? Yeah, but it's still not ideal. In a perfect world, Hard Mode enemies would be close to this generous with heavy ammo naturally, and the dumb heavy ammo armor glitch wouldn't exist. This is just a stop gap.
Idk, guess ymmv.
When I tried it earlier today I had trouble getting my primary ammo not to refill. How much did you empty the clip?
Both primary and heavy need to be at 0. For practical purposes it's better to use something that has low reserve ammo like the Remote or NLB. All the other rifles have hundreds in the bank so they'll take to long to empty.
You also gotta time it so you don't do it too early (your ammo regens after like 20 seconds), or too late (your teammates kill everything, which means you don't get heavy ammo)
Honestly, if everyone on your team does it I can see it working well but good luck with impatient or uncoordinated groups.