Can you put a vert grip on a Ar15 pistol?

Hand stop yes. Vertical grip no. 'Cause ATF.

I was under the impression it was not that simple.

I thought this is where the 26" OAL starts to change the definitions.

So, if an AR pistol is over 26", then it is no longer considered "concealable" and does not fall under the AOW definition if a vertical grip was put on it, which would mean you can put a VFG on it. This would then make the "pistol" into a "non-NFA firearm" which requires no stamp, but is no longer legally a pistol (don't conceal it loaded lol).

If the pistol is under 26", then you cannot use a VFG, but you could use a handstop. I don't know what the difference would be but I would imagine as long as its not obviously a vertical grip, and its manufactured and marketed as a "handstop" I imagine you could at least defend your case. The BCM KAG would be the one that may be close to walking the line, even though no one actually uses it as a vfg.

Someone correct me if I'm off here.