now i know this was cover some time ago but if sombody could refresh my mind that would be great.  

I have the ARMS sir on a couple of my weapons,  i want to install a TD vert grip on it,  I know they are not compatible but was wondering does anyone know what modification has to be done to the grip or where i can find the replacment rail that is out there.  saw it a couple of times befor i had the TD

thanks

S/F

i'm in the same boat as you.  i wrote to td and they said "...You will need to modify the top two inside vertical surfaces of the vertical grip mounting channel. Carefully file or Dremel those surfaces back to approx 45 degrees until the grip clears the lower handguard. Do not cut into the dovetail. Take your time, and remove only enough material to allow your grip to install."  
i still have to mod mine, haven't done it yet.  hope this helps.