Foregrip handles aren’t regulated. But if you had an unused one and a handgun like the PLR that could accept it, the feds might say you have ‘’constructive possession’’ of an SBR. It’s the same logic that lets them bust people for having an AR-15 and certain M16 parts, even if they never install those parts. It’s illegal to have the ability to do something illegal.
It’s dumb, and probably pretty safe since unlike M16 parts there are many guns to which you can legally attach a vertical foregrip. But if the ATF wanted you, I think they could get you.
I’ll make it as difficult as I can by attempting to stay legal.