Maryland Laws re firearm transfers
First, I'm no lawyer.
The Maryland firearm laws that I've found are links in
http://www.mcrkba.org/MarylandCode.html
Maryland's regulation concerning straw purchases are given
here
So far as Maryland is concerned, straw purchases pertain to what they call regulated firearms. Scan down the page
here for a definition of regulated firearms in Maryland. Regulated firearms mostly include handguns and "bad" semi-automatic rifles.
I don't believe the rifle in question is a regulated firearm to Maryland. Maryland regulations appear to apply mostly to regulated firearms and less so to non-regulated firearms. Mostly, Maryland defines what is not legal to do. A purchase of a regulated firearm by a Maryland resident outside of Maryland is controlled by the Maryland Code
here and for rifles or shotguns which are not regulated
here.
Maryland code is scattered incoherently in a number of articles. It is easy to overlook a provision. However, it appears that Maryland bans
allowing access to firearms by children (under age 16) except when supervised by a person age 18 or older.
Were your father a Maryland resident who had legally bought the rifle, it appears he could hand it to you and you could legally possess it, but not own it, so far as Maryland law was concerned. Maryland does not permit private sale of regulated firearms (they must be transfered through an FFL or by the State police). I'm not sure, but it may be the same for any firearm.
Because your father isn't a Maryland resident, federal regulations would apply also to the transfer of ownership. It sounds as if your father has already purchased the firearm. If he hadn't, I believe it would have been legal for him to come to Maryland, and accompany you to purchase the rifle in your name under a stipulation you wouldn't be given possession of it until age 18. It is legal for minors to own some firearms in some states, but occasionally isn't legal for them to possess them. He could then have left the firearm in the custody of your Mother until your 18th birthday.
I don't know of any provision of federal or Maryland law that would have prevented that transaction.
I welcome any correction to this understanding -- but would appreciate a link to the relevant federal or Maryland code.