I always seal the inside of wooden stocks with True Oil or LinSpeed but probably doesn't help much since the outside has the original finish.
If I test it w/o the fore arm I'm fairly certain it would hold zero but then what? I would have narrowed the problem down but what to do about it?
The truth of the matter is you can't truly seal a piece of wood from moisture fenestration. Least effective are "oil finishes", most effective is varnish or epoxy but even they aren't 100% fool proof (but close). Wood is dynamic, and will move to one degree or another with climate changes no matter what you do to stop it. How the stock is cut- quartersawn or flat sawn- will figure in to a large degree how much it will move over its life too. I betcha that wood movement is effecting the tension on your fore arm attaching screw whether you want it to or not, especially since that screw is also doing double duty as a sling anchor.
The benefit to shooting it with the fore end removed is that you are totally removing the wood from the equation- giving you a base line for future testing. Rest it on the receiver, or if on the barrel make doubly-damned sure it's resting on the exact same spot for each shot. The trouble inherent in all of our bench shooting is that we are humans and not machine rests.
Have you investigated thoroughly exactly where the wood is touching steel, ie: with transfer agent? Could be it's bearing hard on one side and not so much on the other which will compound any errors caused by wood movement. Is it bearing real tight back against the receiver? There should be a RCH of clearance there- not so much that a gap is visible but not actually touching either. (That's a protocol common to all two-piece rifle stocks, especially single shots and, I suspect, Savage 99's too.)
If after addressing each issue one at a time, followed by re-testing at the range at every step along the way (elsewise how will you know what it was that worked), and are firm in the knowledge that technique is having zero effect on your results, then try the O-ring trick because what else do you have to lose?
Personally, I never rely on/expect a wood stocked rifle to hold its zero come what may during seasonal cycles.
