Well, let me just put it this way - if BATFE is really interested in pursuing someone who legally has vertical foregrips for a specific rifle (which happens to already be registered NFA), which can also be mounted on a pistol with a rail on it (not sure why anyone would ever do this???), they could probably find out who I am and where I live.
If you use something like a shoestring or even cardboard (supposedly it's been done with certain guns no longer in production) to make a semi-auto fire full auto, that's completely different than what I'm talking about. I might have some duct tape and plastic coke bottles laying around that could be illegal NFA items too - but, I don't think BATFE is prosecuting people for keeping duct tape and coke bottles in the same house as gun(s).
I guess what I'm saying is I think there's a lot of fear and hype about what BATFE will do to you, when you aren't breaking any laws.