Opponents of the ban claimed that its expiration has seen little if any increase in crime, while Senator Diane Feinstein claimed the ban was effective because "It was drying up supply and driving up prices."
The Centers for Disease Control and Prevention studied the "assault weapon" ban and other gun control attempts, and found "insufficient evidence to determine the effectiveness of any of the firearms laws reviewed for preventing violence," noting "that insufficient evidence to determine effectiveness should not be interpreted as evidence of ineffectiveness." A 2004 critical review of research on firearms by a National Research Council panel also noted that academic studies of the assault weapon ban "did not reveal any clear impacts on gun violence" and noted "due to the fact that the relative rarity with which the banned guns were used in crime before the ban ... the maximum potential effect of the ban on gun violence outcomes would be very small...."
In 2004, a research report submitted to the United States Department of Justice and the National Institute of Justice found that should the ban be renewed, its effects on gun violence would likely be small, and perhaps too small for reliable measurement, because rifles in general, including rifles referred to as "assault rifles" or "assault weapons", are rarely used in gun crimes. That study by Christopher S. Koper, Daniel J. Woods, and Jeffrey A. Roth of the Jerry Lee Center of Criminology, University of Pennsylvania found no statistically significant evidence that either the assault weapons ban or the ban on magazines holding more than 10 rounds had reduced gun murders. However, they concluded that it was "premature to make definitive assessments of the ban's impact on gun crime," and argue that if the ban had been in effect for more than nine years, benefits might have begun to appear.
Research by John Lott in the 2000 second edition of More Guns, Less Crime provided the first research on state and the Federal Assault Weapon Bans. The 2010 third edition provided the first empirical research on the 2004 sunset of the Federal Assault Weapon Ban. Generally, the research found no impact of these bans on violent crime rates, though the third edition provided some evidence that Assault Weapon Bans slightly increased murder rates. Lott's book The Bias Against Guns provided evidence that the bans reduced the number of gun shows by over 20 percent. Koper, Woods, and Roth studies focus on gun murders, while Lott's looks at murder, rape, robbery, and aggravated assaults. Unlike their work, Lott's research accounted for state Assault Weapon Bans and 12 other different types of gun control laws.
The Brady Center to Prevent Gun Violence examined the impact of the Assault Weapons Ban in its 2004 report, On Target: The Impact of the 1994 Federal Assault Weapon Act. Examining 1.4 million guns involved in crime, "in the five-year period before enactment of the Federal Assault Weapons Act (1990-1994), assault weapons named in the Act constituted 4.82% of the crime gun traces ATF conducted nationwide. Since the law’s enactment, however, these assault weapons have made up only 1.61% of the guns ATF has traced to crime." A spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) stated that he "can in no way vouch for the validity" of the report.