Thanks for all the tips. It's on the forearm, and if I unscrew the swivel all the way out, I can see a nut in there already. I'm not really worried about it pulling out, because it seems solid, but the swivel is screwing in too far, wearing away a bit at the stock which used to stop it, so that it goes an extra 1/6 turn, and making the swivel post off centre. (hopefully that makes sense.
The stock is an HS Precision synthetic, the factory one for a Rem 700LTR.