Is it legal to add a vertical grip to an AR pistol. I have heard that it is not, just wanted to get more info on the subject. I hav a Springfield Armory Saint Edge pistol. According to the specs on their website, it is 28.5." fully extended including the stock flash hider.

The flash hider is not included the all length unless its permanently attached.

This might be the most current discussion:

https://www.ar15.com/forums/AR-15/Best-way-to-achieve-26-OAL-on-a-10-5-Barrel-/122-738177/

but this topic comes up every few days in the AR Pistol forum. You can read around in there for good info, and some good pics for ideas on your build.

Also see this one:

https://www.ar15.com/forums/AR-15/ATF-and-length-of-pull-/122-737487/

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