I don't know about "todays rules" but in the olden days leaving it in a safety deposit box was common. It hasn't left your possession because your the one with the key. Deploy to Iraq, lock it up 'til you get back sort of thing.You cant legally do this either. The person on the Form 4 has to keep possession of the Class 3 item the entire time. You are also required to notify the ATF when the address on the Form 4 where the Class 3 item resides changes as well. Failure to do either of these can result in the Class 3 item being removed from your possession and the Foom 4 revoked.
The OP is very limited here if the Class 3 item in listed under a singular persons name, he either keeps it with him and notifies the ATF and files the correct Form to update the address (I think it is a 5510, been awhile since I did mine) to his new address or else sell the Class 3 item and go thru the process to have a Class 3 dealer hold the item until the new owner gets their Form 4 back with the new stamp.
When MG's got expensive buyers and sellers would open a box together where neither one could get at it til the transfer went thru. That way the seller couldn't disappear with the gun and the money. But times do change !