First, huge numbers of high-capacity magazines are already in circulation. Under the last ban, the price went up, but they were still available, and more have been made and sold since. Even if sales of existing magazines are forbidden, they’ll still exist, and change hands quietly. That is, aside from the ones that people are already manufacturing on hobbyist 3D printers or in metal shops. Getting existing magazines out of circulation is a non-starter, since nobody knows where they are and most owners are unlikely to surrender them when keeping the things is essentially a risk-free enterprise.
Which is the same problem faced by the “universal background checks” Biden insists are part of the emerging consensus he perceives among the people who already agree with him. The background check brainstorm is a bone thrown to people who heard somewhere about a “gun show loophole” — not realizing that most private owners can sell free of paperwork requirements anywhere, in the majority of states, while commercial dealers have to do background checks, even at gun shows. Americans own an estimated 270 million firearms (PDF), most of them unregistered. Even records in those few states that require some sort of registration are compromised by the fact that owners move out of state, or in-state from elsewhere, and the lists become inaccurate and unreliable over time. A gun owner in New Jersey, for instance, where multiple levels of paperwork are maintained, could move to bureaucracy-free Arizona, then move back to Trenton (for reasons I could never fathom) and plausibly deny still owning any of the guns the state of New Jersey meticulously recorded.