[Ed. – Basically, the ATF rules change would apparently make interpretation more arbitrary for classifying pistol-length weapons designed to be used with bracing devices. Owners wouldn’t have reliable specs to comply with, because ATF doesn’t propose to state them (only listing what will or might be taken into account): problematic over time, and likely to produce, on a large scale, an initial, highly arbitrary reclassification of weapons currently classified as pistols; i.e., making them short-barreled rifles (or shotguns). Timing: Congress probably can’t prevent this before it would go into effect in January, due to the Democratic majority in the House, and Trump could override enforcement, but Biden wouldn’t.]
The ATF published a notice Thursday that could require millions of AR-15 pistols and similar firearms—which are designed with braces that strap on to a shooter’s forearm—to be either registered, turned in, destroyed, or dismantled. But the standards laid out for determining the devices’ legality, such as caliber or weight, provide no objective measures, and the agency said it may also use undisclosed factors to judge the legality of the devices.
The agency conceded in the notice that some pistol braces are legal and should not be subject to the registration or destruction requirement. It said, however, that it could not provide a blanket determination for which pistols, or braces with which they’re often equipped, are legal and said it would have to examine each gun “on a case-by-case basis.” That means owners of the vast majority of the estimated three to four million AR-15 pistols and similar firearms may have to register with the ATF.