Omar Mateen, the Orlando shooter, used an AR-15-style semi-automatic rifle and a 9 mm semi-automatic pistol to kill 49 people at the Pulse nightclub Sunday. The same style of rifle was also used by the shooters at San Bernardino, California; Umpqua Community College in Oregon; Sandy Hook and several other mass shootings in recent years. On Monday, Democratic presidential candidate Hillary Clinton described the rifle as a “weapon of war” that should be banned, and President Obama concurred. But an effective ban could be difficult to devise.

The AR-15 used to be illegal. President Bill Clinton’s assault weapons ban, which was in effect from 1994 to 2004, banned the AR-15 and other guns that were too similar to military-style weapons. However, this law did not prohibit Americans from owning semi-automatic weapons; it capped how many military features an individual gun could have. During the ban, a semi-automatic rifle like the AR-15 could legally have any one of the following features, as long as it didn’t have two or more of them: a folding stock (making the gun slightly easier to conceal), a pistol grip (making the weapon easier to hold and use), a bayonet mount, a flash suppressor (making it harder to see where shots are coming from), or a grenade launcher.

A 2004 report commissioned by the Department of Justice on the effects of the assault weapons ban concluded that the law was largely ineffective at limiting access to weapons with the power of the AR-15. According to the report, the ban focused on “features that have little to do with the weapons’ operation, and removing those features is sufficient to make the weapons legal.” The report noted that several semi-automatic rifles were functionally equivalent to the AR-15 and untouched by the ban. It’s hard to know whether Mateen’s AR-15-style weapon would have been covered by the old ban, though some versions of the Sig Sauer MCX rifle he used are sold with more than one of the components that were limited by the law. Depending on how many military-style features the rifle had when he bought it, it might have been legal under the assault weapons ban. And he would have been able to modify the gun himself, even under the old law.

The review for the DOJ concluded that bans on specific models or features of assault weapons had little to no discernible impact on gun deaths. If the law had any effect, the report said, it was most likely the result of bans on large-capacity magazines, which contain 10 or more rounds. (Large magazines allow shooters to keep firing without pausing to reload, a point at which their targets could run or fight back.) Calculations based on homicide reports in Jersey City, New Jersey, suggested that restricting large-capacity magazines might lower the number of gunshot victims by up to 5 percent. However, there are a huge number of high-capacity magazines already in circulation. The report authors concluded that a ban on them probably wouldn’t make it hard to keep a determined shooter from legally buying a pre-ban magazine and pairing it with an AR-15 equivalent.