Here is an example of a one-gun-a-month law in Virginia…

Studies show that handguns sold in multiple sales to the same individual purchaser are frequently used in crime.  ATF crime gun trace data revealed that 22 percent of all handguns recovered in crime in 1999 had been transferred to a purchaser involved in a multiple sale.  Crime gun trace data from 2000 showed that 20 percent of all retail handguns recovered in crime were purchased as part of a multiple sale.

One-gun-a-month laws prohibit the purchase of more than one handgun per person in any 30-day period.  A study of Virginia’s one-gun-a-month law showed that the law was effective in reducing the number of crime guns traced to Virginia dealers. Virginia initially adopted its law in 1993 after the state became recognized as a primary source of crime guns recovered in states in the northeastern U.S.  After the law’s adoption, the odds of tracing a gun originally acquired in the Southeast to a Virginia gun dealer (as opposed to a dealer in a different southeastern state) dropped by 71% for guns recovered in New York, 72% for guns recovered in Massachusetts, and 66% for guns recovered in New Jersey, New York, Connecticut, Rhode Island and Massachusetts combined.

Source: