The Firearms Blog on 18th December reported on moves to ban “bump stocks” in the US under federal law. A bump stock was used by the killer in the October 2017 Las Vegas shooting that saw 58 people killed and hundreds of others wounded when attending a country music concert. The New York Times has explained that a “bump stock” replaces a rifle’s standard stock, which is the part held against the shoulder. It frees the weapon to slide back and forth rapidly, harnessing the energy from the kickback shooters feel when the weapon fires. It is said to turn a rifle into what is effectively a machine gun.
https://www.thefirearmblog.com/blog/2018/12/18/bump-stocks-federal-register/
http://fortune.com/2018/12/18/trump-bans-bump-stocks-machine-gun-equivalent/
For background on bump stocks, see –
https://www.nytimes.com/interactive/2017/10/04/us/bump-stock-las-vegas-gun.html