In a letter reportedly written to a small home defense shotgun maker, the nation’s top gun control regulator says shouldering a Sig Sauer-made SB15 pistol stabilizing brace could change a firearm’s classification rendering it subject to bureaucratic and expensive National Firearms Act rules.

The Nov. 14 letter was written in response to Black Aces Tactical owner Eric Lemoine’s submission of a short-barreled shotgun design that incorporated the SB15 brace. The brace is intended to allow a shooter to fire an AR-style pistol with one hand using a Velcro strap to attach it to the arm.

The ATF letter has many AR pistol owners concerned as it could be seen as a blanket determination on illegal use of a Sig Brace, subjecting violators to years in prison and a permanent ban on firearms ownership.

“The submitted weapon, as described and depicted above … is not a ‘firearm’ as defined by the NFA provided the SicTac SB15 pistol stabilizing brace is used as originally designed and not used as a shoulder stock,” wrote Acting Chief of the BATF’s Firearms Technology Branch Max Kingery. “However, should an individual utilize the SigTac SB15 pistol stabilizing brace on the submitted sample as a shoulder stock to fire the weapon from the shoulder, this firearm would then be classified as a ‘short-barreled shotgun.’ ”

Black Ace Tactical owner Lemoine told Shooting Sports Retailer the ATF letter applies solely to the shotgun design submitted to the Firearms Technology Branch and doesn't apply to uses with AR pistols.

See copy of relevant passage below...

“When it comes to the AR stuff, I don't know anything about it,” Lemoine said. "We don't follow the AR market at all. I've fired an AR twice in my life. When it comes to questions of whether ATF is walking back its ruling, I don't know what to say about it because I didn't know they had made a ruling on the accessory."

"The only thing I can say is that the letter states that this applies specifically to this design only. ... Just our shotgun," Lemoine added. "If it's an AR, it doesn't mention it in that letter at all."

But some worry Kingery’s note may nevertheless be the beginning of a shift in the ATF’s thinking on the Sig Brace. In March, the agency made what was arguably a groundbreaking ruling on the use of the brace, saying shouldering an AR pistol with an SB15 would not change it to a short-barreled rifle that requires separate registration and a tax stamp.

“Accessories such as the Sig Stability Brace have not been classified by [ATF] as shoulder stocks and, therefore, using the brace improperly does not constitute a design change,” ATF wrote. “We do not classify weapons based on how an individual uses the weapon.”

Since that March letter, the market for AR pistols has exploded, with manufacturers, retailers and shooters seeing a new opportunity to engage with shorter barreled AR-style pistols fitted with a Sig Brace without having to deal with the headache of SBR registration.

Still the ATF’s latest letter to Black Aces Tactical may be a shot across the bow at pistol makers and shooters who use the brace as a makeshift stock.

“The BATFE is in the process of reversing its position on shouldering arm braces due to a change in leadership. It might not be this moment, but it'll be soon,” a worried shooter said on an Internet forum post about the ATF letter. “Investing a ton of money in weapons that are only usable with ‘arm braces’ may not be a wise plan for the future.”