The world can be confusing at times.
If you own an AR pistol and you put an arm brace on it you may have been tempted to throw it up on to your shoulder. Most of us Range Lawyers had an opinion on whether or not that converts a pistol to an NFA short barrel rifle requiring a tax stamp.
Most of you also know there has been a series of letters going back and forth between the BATF and concerned citizens and corporate lawyers.
So far, this seems to be the clearest presentation on the current BATF letter dated March 22 2017. http://www.breachbangclear.com/brace-yourself-for-atf-opinions/
It’s interesting reading as you get an understanding of government organization mindset and a bit of the history. If you own an AR pistol you might want to read it to keep yourself out of trouble as well as to help explain why it may not be illegal to shoulder your AR pistol. I can think of at least one range where a good understanding of BATF's letter would be very useful.