Title nearly says it all.
I noticed that, when I am at 0 bullets with the Raid LMG and use a heavy synthesis, that the LMG will not reload itself, until I switched to the weapon to start the reload sequence.
I can cancel the animation and switch back to another weapon afterwards however, and the LMG will reload itself after a given time.
I think that this is a bug.
If it's an intended mechanic, the perk description definitely doesn't imply this reaction in any way at all.
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If the weapon has no ammo, then cocoon will not proc until it has reloaded even once.