For the time being, I think you might be SOL...
If the loss of the weapon were due to a glitch in the game or a disconnection, I could understand Bungie needing to replace it.
While I feel your pain (I accidentaly dismantled my absolute favorite Pulse Rifle once...), this appears to be an unfortunate case of user error. I'm not sure that Bungie is going to help you on this one, mate.
To reiterate, I'm not shitting on your issue, I definitely feel your pain though.
Did that once too, I got an exact duplicate of my favorite pulse and accidentally dismantled my leveled up one instead of the new one. A lock button would be awesome.