A post on Shooting Sports Retailer mentions a ruling on using the SB15 Sig Brace and that shooting it from the shoulder is a no-go. However this applies only to shotguns.
“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 Aces Tactical makes tactical shotguns with stocks. They were unaware of the previous ruling the ATF made about the use of the SB15 and AR pistols.
People are now concerned if the ATF might revoke their previous ruling of the Sig Brace. We shall wait and see.