Hi - my name is Kirk and I'm a Criminal Law litigation attorney here to assist you.
It doesn't matter whose gun this is, or that it wasn't his, because there's no way he could have a gun registered in his name (since he's a convicted felon).
He can certainly explain that the gun was in the car and he didn't know it, and that may help soften the punishment he's looking at, but this isn't going to get him out of trouble.
This is going to be tough to prove, especially if the gun was in plain sight of the officer or easily discovered. If it was easy for the officer to find it, it should have been equally as easy for him to find it.
Your husband's better argument may be that the search was illegal/without probable cause. If he can prove that the search was not legal, the discovery of the gun would be a discovery from the illegal search, which would be inadmissible as evidence.
That's a much better angle to take because it's going to be hard to convince the judge that he didn't know it was in there.
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