That's actually the point of /equip ammo. Not /equip ammo "ammo name here" but /equip ammo. What that does, since you didn't specify an equipment piece, is remove your ammo slot completely. So if there's anything in it, now there's nothing. Removing/putting in ammo doesn't lose TP, so it saves you the headache of accidentally shooting your bullet simply because you don't have it equipped anymore, even if you ran out of other ammunition.