The attempted assassination of former US president Donald Trump with a semi-automatic rifle illustrates once again how easy it is for shooters in the US to get their hands on inexpensive, easy-to-use, military-style weapons. The AR-15-style rifle is one of the most popular guns in the US. It has also been a common denominator in some of the country's worst mass shootings. The AR-15 is a semi-automatic weapon, meaning a user can fire multiple shots in quick succession. Its cousin, the M-16, has been used by the US military since Vietnam. While some military assault rifles are fully automatic, civilians are prohibited from owning such weapons in most circumstances.