It is called constructive intent. If I had a 10.5" AR barrel and no lower that could legally take it, the ATF could charge me with constructive intent to build an unregistered SBR.
Having the parts to build something that would be illegal, even if you haven't yet, is illegal.
Since all I have are post ban AR-15 lowers, if I was in possession of a 16"+ barrel that wasn't of a heavy profile, MD could potentially charge me with constructive intent as I don't have anything that could legally take it (and things that could illegally take it).
So buying a magazine for a weapon configuration you don't own, but can be used in a weapon configuration you do own and happens to then have a capacity of more than 10 rounds, MD states attorney is probably going to argue you bought a 30 round magazine, which is illegal to do in Maryland.
If you have a Beowulf, buy Beowulf mags if they are labeled as such as you can find someone who will seek or ship them to you. Otherwise take a trip across the boarder and purchase them and bring them back. Stay within the law.
Great explanation, thank you. It's kind of a moot point since I doubt any shop would sell such a thing, I was just curious.