In a stunning reversal in less than a year, the nation’s top gun regulator has ruled that shouldering an AR pistol using  a popular accessory designed for use with one hand is illegal and could turn thousands of gun owners into felons, legal experts say.

On January 17, the Bureau of Alcohol, Tobacco, Firearms and Explosives issued an open letter saying that shooters who build an AR-style pistol using the SB-15 Pistol Stabilizing Brace and fire the pistol from the shoulder are in effect “redesigning” the pistol into a short barrelled rifle which is subject to a special license and tax samp.

In March of last year, the ATF said in a publicly-released letter that shouldering the SB-15 brace, distributed by Sig Sauer, was perfectly legal and did not turn a pistol into a short barrelled rifle.

But the new letter throws out the ATF’s previous opinion.

“The pistol stabilizing brace was neither ‘designed’ nor approved to be used as a shoulder stock, and therefore use as a shoulder stock constitutes a ‘redesign’ of the device because a possessor has changed the very function of the item,” the January 17 ATF letter says. “Any individual letters stating otherwise are contrary to the plain language of the NFA, misapply Federal law, and are hereby revoked.”

“Any person who intends to use a handgun stabilizing brace as a shoulder stock on a pistol … must first file an ATF Form 1 and pay the applicable tax because the resulting firearm will be subject to all provisions of the [National Firearms Act],” the ATF says.

The move has stunned many in the firearms world for its firmness and comes after months of similar letters written to individual firearms and accessory makers that hinted a reversal of the bureau’s previous ruling on shouldering the Sig Brace was imminent.

Some gun advocates dismissed the earlier ATF letters, arguing they didn’t foreshadow a broad ban on using an SB-15 brace as a shoulder stock. Still others warned of an inevitable closing of what many thought was a short barrelled rifle “loophole.”

“People have been saying that the ATF is about to do a 180 degree about face on the idea that using a pistol arm brace as a stock is perfectly legal, and it appears that the day has come,” wrote Nick Leghorn at the popular Truth About Guns website. “There’s no ambiguity this time: using a pistol arm brace the ‘wrong’ way makes the gun an NFA weapon, according to the letter.”

While the new ruling from ATF could change otherwise legal gun owners into felons overnight if they are arrested for shouldering what was for the past year a perfectly legal pistol, firearms law experts say the bureau’s logic is unlikely to stand up to any court challenge.

The key, legal experts say, is that the ATF is essentially saying that by misusing something — putting an arm brace to your shoulder — the shooter is “redesigning” the item into something else.

“If an individual builds an AR-type pistol using a SigTac SB-15 brace, uses it in the manner for which it were (sic) designed and then hands it to a friend who shoulders it, did the friend just ‘redesign’ the brace?” wonders Adam Kraut, a firearms lawyer with the Prince Law Offices in Pennsylvania. “According to this letter they just did. So is the firearm no longer a pistol and now a SBR?”

“I don’t know of a single person who would think that “redesign” entails the misuse of an object,” Kraut adds. “If anything, I would venture to say it would require the individual to modify an existing object. If I were to use a screwdriver to pry open an object did I just redesign it? Is using a pencil to drum on the table redesigning it into a drum stick?”

It is unclear how this ruling will affect the exploding AR pistol market, which has blossomed since the ATF’s March ruling saying shouldering the brace wasn’t illegal. Clearly many shooters are now at greater risk of arrest if they use their SB-15 brace for anything other than one-handed shooting.