Geez Rex, you have my sympathy. It sounds like a gambling addiction for sure! That's the reason why so many states used to keep out the casinos, etc. Now their governments just see taxes coming in and don't care about the lives which are ruined.
Okay, I'm off my soapbox. It is your decision what to do. You can contact the authorities and try to get the guns back, or your can ignore it for the sake of family piece. If he is making life hell for your mom and the rest of the family, your turning him in could be what he needs to straighten up. It will probably make an end to your relationship with him, though. On the other hand, if you do nothing, he knows what he did which will also put a strain on the relationship.
Again, you have my sympathy. You have a tough decision to make here!
-Kees-