From 18 USC 921(a)(31): "The term 'large capacity ammunition feeding device' - (A) means a magazine, belt, drum, feed strip, or similar device manufactured after the date of enactment of the Violent Crime Control and Law Enforcement Act of 1994 that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition; but (B) does not include an attached tubular device designed to accept, and capable of operating only with, .22 caliber rimfire ammunition."
And now 18 USC 922(w): (1) "Except as provided in paragraph (2), it shall be unlawful for a person to transfer or possess a large capacity ammunition feeding device. (2) Paragraph (1) shall not apply to the possession or transfer of any large capacity ammunition feeding device otherwise lawfully possessed on or before the date of the enactment of this subsection."
Bottom line: Since it wouldn't have a detachable magazine, it wouldn't qualify as an "assault weapon" under 18 USC 921(30)(C), but your post-ban magazine is still illegal, even if it's permanently attached.
My $0.02.