The AR-15 rifle, pictured above, is among the weapons banned by the Maryland law. (Photo illustration by Joe Raedle/Getty Images)

Nearly two dozen states are supporting a legal effort to overturn the assault weapons ban passed in Maryland in the aftermath of the Sandy Hook elementary school shooting.

The law, signed by Gov. Martin O’Malley in May 2013, is among the nation’s strictest gun-control measures, banning magazines that hold more than 10 rounds of ammunition and most semi-automatic rifles, while also mandating fingerprinting and training for some new gun buyers. Violation of the law carries a maximum penalty of three years in prison, a $5,000 fine or both.

The law was challenged by opponents shortly after passage and upheld by a federal judge in August. That decision is currently being appealed before the Fourth Circuit Court of Appeals, where the 21 states laid out their case against the law in a filing last week. All but a handful of the state attorneys general are Republicans.

In their filing in support of the challengers, the states lay out four arguments, the first of which is that owning the prohibited weapons is constitutionally protected.

“The weapons banned by the Maryland law fall within the protection of the Second Amendment because they are typically possessed for lawful purposes, including the core protected purpose of self-defense,” they argue.

In her August ruling, U.S. District Judge Catherine C. Blake found differently: “The court seriously doubts that the banned assault long guns are commonly possessed for lawful purposes, particularly self-defense in the home, which is at the core of the Second Amendment right, and is inclined to find the weapons fall outside Second Amendment protection as dangerous and unusual.”

Second, the 21 states argue that law “burdens the core right” of law-abiding citizens to defend themselves. In their third argument, the states urge the court to overturn the law because it fails to satisfy the standards courts use to weigh laws that may infringe on constitutional rights. Finally, they argue the August decision mistakenly accepted expert evidence by criminology professor Christopher Koper.

The law is being challenged by nine plaintiffs, including individuals, a retailer and gun rights groups.

The 21 states are: Alabama, Alaska, Arizona, Florida, Idaho, Kansas, Kentucky, Louisiana, Michigan, Missouri, Montana, Nebraska, New Mexico, North Dakota, Oklahoma, South Carolina, South Dakota, Texas, Utah, West Virginia and Wyoming.

Read more:

The 21-state filing

Judge Blake’s August ruling

Maryland assault weapons ban upheld in federal court