Michael -- something I've played with just a bit, but not enough to say yea or nay:
Instead of making the bag & it's sand more soft & flexible, consider putting something between the bag and the pedestal. I've tried some foam with an old Hohen top -- the thin bag & outboard clamps make this easy. And with a Farley joystick Mark I (the older one), where the welded sides of the container also make it easy.
But really, you could rig something with any pedestal & top, I'd think. This is going to keep the forearm from bouncing so much, if indeed it does bounce, and if bouncing is a problem...
The foam I used was very hard. About as hard, or maybe harder, than the cheap flip-flop sandals they sell for $1.98 every summer. Too hard, I think. I never got back to trying different materials. A dense close coat foam was on the list, never got to it. But the notion of letting something other than rule-regulated sand do the heavy lifting is appealing.
Another thing I noticed, with my 30s -- .30BR, 30PPC, and a long-range 1000 yard 17-pound-almost-magnums -- was I got less vertical with the older pedestal/bag combinations. The worst was the Farley & and Edgewood bag with the stiffener in it. Except sometimes it wasn't. And two of those rifles were barrel blocked -- the .30 BR and the long range .30. The .30PPC is a glue-in. That might matter too.
I do believe this is going to turn out to be one of those experimental things, each rifle & perhaps bedding system etc. giving different answers. It will turn out there is no "best," if by best someone means brand of pedestal, bag, or even type of sand.
With testing bags, you're waiting for an outlier rather than a pattern, but I still believe you can use four-shot groups to test, with a 10-20 shot group to confirm.
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BTW, still waiting for an answer on how to swage flash holes to a consistent diameter. You posted a picture of your tooling once, but I was too dumb to figure out how it worked. Just maybe a lot of words along with the pictures? Might be able to make the tooling if I knew what I was trying to do... I'm a firm believer that ignition doesn't stop with the primer going off.