I hate the idea of having to buy ammo just to use a gun, or having to pay for every round of ammo; it seems like it would make things pointlessly tedious. I would however like to have a variety of optional exotic ammo types to equip and use though. There a way to have it both ways.
When reading this, keep in mind gun customization is coming soon. Anyway, this is how it should work.

All guns should come pre-fitted with a FREE basic ammo pack, much like how like how a vehicle comes with free turrets already equipped. This means if you don't care about ammo types and all that, and you just want your gun to work, then you don't have to have to buy ammo.

When a gun (and the rest of the fitting) is destroyed in battle, so is the ammo pack as well. If the fitting survives the battle then the ammo pack should stay even if all the shots were all depleted in the battle; a surviving ammo pack should be resupplied back to its maximum capacity after the battle ends (since nanites can just replenish them).

Non-basic ammo packs should not come pre-equipped in a gun, but should have to be purchased seperately, and fitted into the gun in place of the basic ammo pack.

Nanohives should still work as usual, and replenish ammo of any type (basic, or non-basic).

Purchasable non-basic ammo packs could include, but not limited: anti-shield rounds, anti-armor rounds, extra range rounds, faster swarm launcher missiles, incendiary mass driver rounds to light things on fire, stasis webifier mass driver rounds to slow enemies. There could also be non-basic ammo packs for higher round capacity and no other special effects.

EDIT: Like it has been pointed out, this idea is basically having ammo packs as weapon customization modules, and not as a regular depleting resource like bullets in real life. Bullets are easily replaced by manufacturing nanites (like ones in nanohives), so it wouldn't make sense to have them work like real life bullets. This means if you finish a battle with an ammo pack not destroyed in the battle, but depleted of bullets, next time you deploy in a new battle you will start off with a full pack of ammo (yay for nanites).

Thanks for reading.