If that's the letter were discussing, there is no way I would say a completed Lower with a M4 stock attached is ok for a pistol build.
The letter says a M4 receiver extension in itself is ok for a pistol build.
To be safe, I would not build a pistol from a rifle lower, just too many gray areas!
Found the letter I was looking for:
Yes, but read the letter in post #13.
Barreled OR Stocked.
The barrel most likely applies to most other firearms, where the barrel directly installs into the receiver/action. Unlike the AR, where the barrel attaches to the upper, which is NOT the serial numbered part.
Also, what was the date of that letter?
and the pdf i posted could superceds both of those.
And if a stock was available to you, you'd could be looking at constructive possession.
Too many ways to get into trouble here.
If you didn't also possess a rifle upper, you could still be charged with constructive possession unless the pistol either has a dedicated pistol buffer tube which could not accept a stock or the buffer was modified so a stock could not be installed. If you had a carbine stock and a rifle upper and the lower started as a pistol, it could be legal.
I don't want to find out what BATFE would decide to do. be our guest though.