It might just be that the rifles in question are only about as accurate as the results you've been getting, and parallax has nothing to do with it. At what distance are you shooting, and with what scope? Are you actually seeing parallax through the eyepiece (ie reticle moving on the target as you move your head around without moving the rifle)?
If you want to try the comb-raising thing though, without permanently modifying your rifles, you could use a bit of neoprene. Easiest of all I've found is what we call a stubby holder here (I think you call them "koozies"). You knock the bottom out of it and you have a cylinder of neoprene which can stretch over the butt to give you a little extra height. A piece of neoprene from a mouse pad, attached with tape, will do the same job, and you can add thicknesses with extra layers of neoprene. If that works for you you can go to something a bit more permanent, but if it makes no difference it has cost you nothing.
