After the passage of the AWB of 1994, the percentage of guns seized by the police in Virginia in connection with crimes with high capacity magazines dropped from 13% in 1994 to 9% in 2004. How was this possible, since many high capacity magazines were in criminal hands prior to 1994, and pre1994 magazines were grandfathered, and continued to be available from private sellers?
To know why, it is important to understand three things.
The stock of guns in criminal hands has very high turnover. If 70% of armed robbers are rearrested within three years, the stock of guns in criminal hands has a very short half-life, either through arrest and confiscation or death before another criminal has a chance to loot the body.
During the AWB, criminals needed to replenish their supply of munitions, but during the ban, their supply of high capacity magazines was constricted. Before the ban they could steal new high capacity magazines from licensed dealers or shippers, buy them from corrupt dealers, obtain them through straw purchase or from friends or family who had bought them from a licensed dealer. All of these sources were greatly diminished, although new high capacity magazines for law enforcement or military use remained vulnerable to diversion.
Used high capacity magazines made before the ban were still available from private sellers and licensed dealers in used munitions. However, second hand guns and magazines seem to have been a clear minority of the inventory available at any given time for corrupt diversion, theft, or straw purchase.
Third, guns possessed by criminals are a positional good. If an illegal drug dealer is arrested, the supply of that drug is diminished and other dealers rush to respond to the demand. If a criminal with superior firepower is arrested, other criminals need less firepower to maintain the status quo.