I get the impression that a lot of criminal gangs cycle through a lot of their guns pretty quickly. Guns used in serious crimes are often discarded, people throw guns away when they are being pursued, all that. Some guns remain in possession of street gangs for a long time - but many don't.
Safe storage requirements could reduce thefts that move legal guns into the black market.
Universal background checks could reduce fraudulent or negligent purchases that move legal guns into the black market.
Reductions in the number of
guns purchased at once could reduce
straw purchasing that moves guns into the black market.
Regulations requiring dealers to refuse sales to people who are obviously impaired or that appear to be acting under orders of other people, or at least ensuring that such actions by the dealer would not be illegal could further reduce
straw purchases.
None of this would have an immediate impact, but over time the supply of black market guns could be reduced without resorting to confiscation of guns held by people with the legal right to own guns whose guns were acquired legally.