Asking for a friend. You cannot own or possess an evil "assault weapon" as a resident of Allston, cirrect? That is part of Boston proper?
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Let me put it to you this way: your friend can legally/officially put down his address as either Allston or Boston. I used Boston and Brighton interchangeably when I lived there.
I wouldn't use a mailing address as a definitive factor. When I had a PO Box in Auburndale (part of Newton), the postal employees told me that I could use Boston, MA 02466 . . . apparently they key off the Zip not the name of the town.
To answer the OP, answers above were correct.
However, on a technicality a Boston resident can OWN an AR . . . what is forbidden is POSSESSION within the CITY limits. So if was stored outside the city and the resident never crossed the line into any part of the city with said banned gun, it would be legal to own it and take it elsewhere to shoot.
Legally no FFL anywhere in the US is supposed to sell an AR (or other listed banned gun) to a resident of Boston, but I'm 110% certain that almost no FFLs are aware of this restriction. BATFE Regs say that the FFL is to ensure that possession would be legal where the buyer resides (Boston) before doing any transfer.