If they were to charge him with a federal hate crime, they would still need to prove the actual crime occurred. The "hate" part of that phrase refers to his intent in committing the crime, but they would still have to prove he shot Martin, and for that, they would need the gun.
And to those saying Zimmerman's attorneys could stipulate that is the gun- you can't stipulate to something if there are no ongoing proceedings.
If the DOJ decides to prosecute Z all they have to do is enter a transcript of the State trial in which the defense didn't deny that Z shot TM and they didn't deny that Z shot TM with that particular gun.
The DOJ wouldn't have to prove that Z shot TM.
They would have to prove that Z shot TM just because he was black.
Holder is just using any means necessary to keep Z from getting his gun back because Holder and "his people" can't stand the fact that Z was found not guilty and could again be walking around with the gun that killed TM.