I just bought a sling stud, drilled, and tapped my RRA AR.
Installed one of these
YHM Swivel Stud - YHM9469
In hind sight I should have gone with the fixed (
YHM Fixed Sling Stud - YHM9476) but oh well.
It was a odd thread size and I had to special order a tap. I still have it somewhere if you decide to go that route I would be happy to lend it to you.
Once you have the sling stud then you can either attach a bi-pod that attaches directly to the sling stud or the one I am using has an adapter that adds a short piece of rail and the bi-pod attaches to the rail.