If you can't infuse it or move it to the Vault and it only holds one round then you have the quest version of the gun. You get a limited version of the gun during the quest because you need to use it to finish the second half of the quest. That quest was vaulted and can no longer be completed. As such, the final version of the gun was added to the Monument to Lost Lights in the Tower. The version you have should be deleted and if you wish to own the final version that can be infused, moved to other characters or the Vault, and holds more than one round just purchase it from the Monument to Lost Lights in the Tower.