Depending on whom you ask, either the ARMS rail is not to spec, or the TD Grip is not to spec, and the TD fits other rails just fine... I instead got a replacement rail from TD that fits and didn't have any problem after that... unfortunately TD is no longer making them. The other option from what I understand is to grind down a little on the TD grip to make it fit. I haven't done that so I am not the person to ask how much work is involved, but it has been posted here before.
Dave