As that process occurred, the value of the weapons would spike as scarcity took hold, and tactically-useful firearms (e.g. semi-auto) would become expensive on the black market. This would mean that criminals would need to be far more judicious with how they carried and used them.
It is very plausible that this scarcity effect would lead to a meaningful reduction in the possession and use of firearms by low-level street criminals, which would also by extension lead to a reduction in levels of firearms-related homicide, assault, and intimidation.
A low-level narcotics broker is less likely to carry around a Glock that costs $10,000 (which they have to dump off of a bridge or in a storm drain every time it's used in a homicide), than they are to carry around a black-market stolen Glock that cost $600.