This has always intrigued me too. I have a really fancy rest, a 4x6 block I throw two folded towels over, I rest the rifle forearm screw roughly centered on this block and shoot away. I get 5 bullets in a hole with my 38-55 or my 357 Max, and when I stand up on my hind feet and bang on a steel plate, the rifle shoots to the same point of aim. This is true no matter if I am using cast or jacketed. I also don't use the o-ring, I simply leave the forearm just a touch loose.
I find the Handi to be very sensitive to cant, Something about the stocks and scope height leave room for the rifle to roll around and scatter the shots about. I do all I can to hold the rifle and crosshairs the same shot to shot and I also set the scope up by shouldering it as I align the crosshairs for vertical. What I notice is that when I shoot off the bench, the rifle needs a touch of cant to make the crosshairs true to the vertical/horizontal. So there is some difference between how I naturally hold the gun when standing and sitting. For shootin' deer at 50 yards and less it don't make enough difference to matter. When shooting for the best I can shoot, I have to watch the cant, or the shots scatter.
I'm not convinced the o-ring does a lot, by the time one has shot enough to get frustrated and begin to seek solutions, one is also beginning to slow down and do more of the small things that contribute to shot to shot consistency that add up to better groups. Do o-rings and how the rifle is help make a difference? Absolutely. How much difference? Only each shooter can say. But a cant to 58 minutes, (just left of 12 o'clock), adds up to 3/4" left and 1/2" high at 50 yards. One or several such shots can make an otherwise good group really lousy.
I saw a leveling devise that can be used with a scope somewhere, don't now remember, but it could go a long way to making ones groups better.
