After being repeatedly used in mass shootings, assault rifles are again coming under scrutiny all while purchase sales soar.

The gunman in the Orlando mass shooting that took place in June of 2016 used a Sigsauer MCX, a military-style assault rifle that is one of the most popular in the country for retailers of firearms. But it is also coming under scrutiny after similar styles of assault rifles have been used in mass shootings across the country.

The Sigsauer MCX rifle was designed at the request of the US Army’s special operations forces, according to Mother Jones, and meets specific standards set by the Department of Defense, according to the company’s own promotional material. Unlike other assault rifles, the MCX is unique in that it was a newly designed rifle rather than a variation of an AR-15 design.

That hasn’t prevented many media outlets from pointing out that the MCX and other AR-15-style assault rifles have become the primary firearms used in recent mass shootings. The AR-15, a precursor to the MCX, was used for mass shootings in San Bernardino, California; Aurora, Colorado and Newtown, Connecticut.

When sold to private citizens in the United States, these military-grade assault rifles are altered to be semi-automatic, meaning that they require a pull of the trigger to fire each shot, but automatically discharge the bullet’s casing and reload the next shot. Some states, like California, limit the amount of bullets allowed in a bullet magazine, but it can be modified. In the case of the San Bernardino shooting, both AR-15’s used were modified in such a way that made them unlawful.

In 1994, Congress passed a federal assault-weapons ban that created a complicated labeling system for what constituted an assault rifle, but it also prohibited the sale of certain AR-15 models, and limited magazine capacity to 10 bullets nationally (with the exception of the 1.5 million assault weapons already in private hands, which were legal and could be sold). The law was riddled with holes, according to The Washington Post, and a University of Pennsylvania study could not conclude the law helped deter mass shootings.

During this same time, a similar movement was happening in Australia. In 1996, the country instituted a similar national law with stricter guidelines and without loopholes in the wake of a mass shooting in Port Arthur, Tasmania, in which 35 people were killed. The country spent hundreds of millions of dollars buying up the approximately 600,000 weapons owned by private citizens, which has been linked to a decrease in gun crime over time. In 2004, the United State’s assault-weapons ban expired and was not renewed.