Legally, a HG is defined as a gun designed to be fired w/ one hand. An AR pisotl is thereticaly so, it just so happens, that same as your glock, it is easier to shoot using the support hand too... The sig brace is specifically designed to help single hand shooting so it is clearly not intended to be used as a shoulder stock.
Seems like it would slide to and from no the receiver extension, if you were to shoulder it as manufactured, seems like it would just slide forward and let the end of the receiver extension hit you in the shoulder anyway.
i believe there have been people who have been prosecuted for using mag holder accessories or other things tha tattach to the receiver ext as a butt, whether or not they were orignally intended such or they modified them, b/c, the ATFags argued that the possesor built the gun to be fired w/ two hands and shouldered.
It might be useful as a cheek rest, but seems like a lot of bulk and $$$ for that.
It is legal to use a full length receiver extension on AR pistol. Its not as compact but provieds the LOP that most men need. A car tube is too short for me. It is legal to shoulder the receiver extension.
Also, a regular rifle extension is stronger than telestock extensions, at the place where it attaches to the lower, a point of failure.
If you use a rifle length tube, you can now use a VFG or AFG (if you are into them), if the overall length is over 26" per a recent ruling.
There are people who have letters, to htem, saying it is legal to put a cane tip in there, ostensibly so that is stable when leaned in a corner. But those are a "positive defense" only to the addressee. I am unaware of a general ruling about cane tips.
You might can get a false telestock extension, rifle length tube, but that hosted fake telectocks during the AWB and use it to host the sig brace thing.
I don't think the brace is practical. It would sort of force you to shoot the gun two handed, which is inferior.
Anyway, its all arbitrary BS, fuck congress and their ATF enforcers