Unless your state requires it. You don't need to do anything. As stated it's yours, The Marine Corp doesn't want it, Even if your Uncle did " borrow it long term " which I doubt. He probably picked it up after service because it reminded him of past times. That is a common failure, if someone was in the service, then after death , going through his effects they find a firearm, well he was in the Marines, then this must be his service firearm. In 99 and 9/10 of the time this is the wrong assumption. I have even seen people ask if a pellet gun they found was a persons " service pistol ". It is yours to do with as you wish ( as long as it is legal
). Even, if it was at one time military property, there is no longer any record of it anywhere.