Shannon Stapleton/Reuters/FileCHICAGO — Since Omar Mateen entered a gay nightclub in Orlando, Fla., earlier this month with an assault-style semiautomatic rifle, killing 49 people, the nation has once again debated whether such weapons should be banned. In seven states and a handful of municipalities, they already are. The experience of these states, combined with the limited data compiled during a federal assault-weapons ban that ended in 2004, gives a rough snapshot of how effective such bans are. The answer so far appears to be: not very. The difficulty of controlling weapons across state lines, the ability of the gun industry to adapt its products, and the complexity of enforcing such laws have led to little substantive change in the number of assault style weapons in circulation in places where bans exist, data and expert analysis conclude.