Bob,
Not a gun stock, but I do sandblasting and bead blasting on delicate parts as part of my business. As you may know already, sand and it's dust travels VERY easily, so be very careful where and on what you use it.
I wouldn't do any sand on, or near mechanical parts. That would need some very tricky masking to prevent any posability of sand or dust getting into the mechanicals, but for a plastic stock with all other parts removed from it...
You can try the paint/sand combo, or light sandblasting just inside the butt of the stock to see if it's a finish you can live with.
If your gonna sandblast, use several layers of duct tape to mask off any areas you don't want blasted. If it's not what you want, the paint/ sand combo can be cleaned out with mineral sprits and paper towels - followed by a good scrubbing with warm water with a bit of dish detergent in it. Then a warm water rinse. Blow dry with compressed air. That way, nothing will show of your test after the gun is back together.
And don't use any harsh solvents on the plastic such as lacquer thinners. They not only melt the surface of many plastics, they can soak into almost al forms of plastics and cause them to swell, possably crack after they dry out.
Whatever you do, if it involves sand, make absolutly sure to keep all the mechanicals well away from where your working with the sand. And before you re-assemble the gun, make sure to give the stock a good wash, then blow dry with compressed air after your done. Even if you eventually use sand in a painted finish (and especially if you do any sand blasting), it should be thoroughly washed and blown dry with compressed air after the paint is well dry so that there is no chance of any loose sand or dust left to get in the mechanicals.
Paul.