The Biden administration’s effort to regulate pistol braces was dealt a blow Tuesday after a ruling by a federal appeals court cast doubt on its constitutionality.

The Biden admin rule required gun owners to register pistol braces, which are accessories that can be attached to the rear of a gun to make it easier to aim and fire with one hand.

Second Amendment proponents argued that the braces make handguns safer and more accurate. But gun control advocates argued the braces could be used to lengthen a concealable handgun, making it more dangerous.