I have a Ruger SBH Bisley Hunter...shoots fine for me, but the factory grips are a little loose, aka rotate around the gripscrew. I like their appearance and feel (no, don't want to replace them), just want to keep them from wiggling. What can I do to anchor them in place?

You may try glass bedding the grips around the thru-pin at the butt (under the grips).

5 quick wraps with some duct tape?

Glass bed them to the grip or place locating pins in the grips along the inside edge of the frame in a couple of places. the pins should be epoxied into the grip with only a sixteenth or less standing proud to align the grip with the frame.

The glass bedding is the best route but is likely to make the grips harder to remove (if you get a tight fit).

If you plan to use
glass bedding please use enough release agent or they well be really hard to remove.

It might be possible to fit some blocks inside the frame that could be epoxied to the grips.

That would keep them from moving, but still allow easy removal

It seems odd they can't just be tightened enough to keep from moving

I will go with the suggestions to glass bed the grips, using release agent, yes. I have glass bedding material on hand. But the duct tape sounds like a quick if messy fix!

I would put a couple of pins drilling frame and the inside of the grips .If that works fine but if not glass bed them .
Jim Cirillo told me about the time he had broken grips and just used tape to hold them together ! The BGs noticed that ! He was in NYPD Stake Out Squad !