[bd111]

It's externally visible, ergo forbidden. What you can do that I think is OK under the rules is modify the stock on the inside, just behind the standard sling swivel, with a threaded backer glassed in to accept a screw-in UM swivel for when you want to use your bipod. Make a small wooden plug that press-fits into the hole flush with the bottom of the stock for when you have to be "legal."