Instead of an angled foregrip, I just want to use a front strap to enhance backward pressure against a single point sling as sort of a finger stop on a horizontal foregrip.  I cannot find an atf tech branch letter on this.  Some say it can dangle so it is  vertical foregrip.  The search function hasn't helped.  Links or examples of the atf recent written opinion is being sought.  thanks

http://www.ar15.com/forums/t_3_122/525036_Request___Scans_of_any_letters_from_the_BATFE__that_pertain_to_AR_pistols.html  I did find it here.  Flexible hand strap is legal.