The 1994 federal assault weapon ban was in effect for ten years and made it illegal to manufacture "assault weapons" for private citizens. It also set a limit on only ten bullets to a magazine. The Democrats were supporters of this law. It expired because the original law was written so it would expire after ten years and congress didn't see any reasons to keep it in effect. It also didn't have any major effect in reducing crimes or mass shootings.

http://www.washingtonpost.com/blogs/wonkblog/wp/2012/12/17/everything-you-need-to-know-about-banning-assault-weapons-in-one-post/