Our Friends over at TheFirearmBlog have the full letter here but let me give you a quick rundown from GAT Daily before you go read the letter.
First a brief brief history.
in 2012 the first brace hit the market and it was quickly picked up by Sig Sauer and became the Sig Brace. People loved how it changed the awkward aesthetics of the AR type pistols into a more useable platform. Obvious to all was its utility to stabilize a firearm in a similar manner to a shoulder stock, giving the firearm the utility of an SBR without the arduous wait time.
in 2015 right before SHOT Show and after several braces had hit the market the Bureau of Alcohol Tobacco Firearms & Explosives (BATFE/ATF) had received literally thousands of letters addressing the brace’s legal utility. This constant cajoling led the ATF to augment their opinion on the whole brace technology to indicate that shouldering the brace constituted a deliberate modification and thus created a NFA item. This was legally challenged by SB Tactical on the basis that while the ATF considered shouldering the brace a “modification” no actual physical modification was done by deliberately shouldering the brace. It was the ONLY piece of ATF approved technology that could be modified without actually being modified, only used unconventionally.
The newest letter from the ATF acknowledges this and effectively reverses the 2015 decision. While the ATF points out they do not like people shouldering the brace they go on to clarify that as long as the brace remains in factory ATF approved condition and undergoes no modification by the end user to increase its utility as a stock (or any modification at all) the brace’s use on a pistol and any use of the pistol while the brace is on the pistol do not constitute a modification of the pistol into an NFA controlled firearm.
They go on to point out that deliberately installing the brace at a length of pull outside of the norm and usable only as a stock would constitute a modification.
So in short as long as the brace is installed as intended from the factory it does not matter how you use your pistol. Just do not modify your brace from factory specs.