If Mr. Manifesto altered the magazine release lock to accommodate detachable magazines as this article hints, that would explain things.
Buffalo Shooter Saw New York's Gun Laws As His Advantage (msn.com)
"In the manifesto, Gendron allegedly wrote about how he planned to "kill as many Blacks as possible" in the attack and decided to carry it out in New York due to its strict open carry gun laws, which would make it harder for someone to stop him, the Buffalo News reported."
Did everyone get that? If not, got back and read it again.