First off, let me preface this by saying that ATF opinion letters are just that. They are the opinion of whomever is working at the Firearms Technology Industry Services Branch whenever said letter comes in. The letters technically only apply to the people who asked the ATF the question and only for firearms and products directly referenced in the letters.

That said, ATF letters have long been used as a window into what the ATF considers legal/illegal. Since they are the federal agency charged with arresting people for violating gun crimes, people tend to pay attention to these letters.

Earlier this year the ATF issued a letter saying that shouldering an AR-15 style pistol with a Sig SB-15 arm brace attached was not illegal. This is what most people have always thought. Use does not change intent and the laws surrounding short barreled rifles specifically mention design intent.

However, several recent letters, including the one that just emerged in the last 24 hours, tends to show that the ATF is changing their tune on shouldering arm braces.

Here is the letter in question which was posted to AR15.com earlier today (however the letter seems to be from November). Note paragraph 5 of the 2nd page:

Last week, the ATF approved a new AR pistol arm brace, but in the approval letter it states that the new brace is only legal as long as it is not shouldered:

Here is another recent letter in which the ATF said that it was legal to attach the brace to a shotgun with a shortened barrel, but that it was illegal to shoulder it:

It would seem that all recent letters to the ATF are getting a similar response in regards to braces – the ATF isn’t cool with it.

This would seem to show that the ATF might be changing their mind because they previously issued a letter on the Sig SB-15 saying that shouldering it was just fine:

Once again, these are just opinion letters. The real test would come in a criminal court if someone was caught shouldering an AR pistol and arrested.