To summarize: if one has an AR Pistol and the total length of that pistol (not including the muzzle device unless it's perminatly attached via one of the approved methods) is 26" or greater, one may add a vertical fore grip and the AR pistol magically becomes a "firearm" in the eyes of federal law. State law may vary. I think the last I heard angled fore grips and handstops were GTG on any length pistol, but I'm not super sure.
If, however, your AR pistol is shorter than 26" and you add a vertical fore grip you have created an "any other weapon." AOWs are regulated and to legally build one you need to file paperwork and buy a
$5 $200 stamp. It's a $5 stamp to transfer an AOW, $200 to build. $200 to build or transfer a SBR.
If you want an AOW or SBR see this thread:
https://www.ar15.com/content/page.html?id=489