When I see "zero shrinkage" without an actual measurement my Engineer brain disregards that as marketing tripe. I know that they all shrink. I am not sure that it is possible for such a compound to not shrink at all. How much it does shrink is the question.
I have experienced exactly that level of fit using the Devcon 10610 AL putty. Each time its caught me off-guard and I go "Oh Berkelely! I hope that's not bonded/locked.... " and it's not, but it is good for getting the heart rate up! No mechanical lock or bonding, but hard to remove the action from the stock. On all 3 rifles that I've used it on. Which is far from being a statistically valid sample size....
Devcon 10610 is a repair putty, so while not specifically designed to bed rifle actions, it is designed to work in exceedingly similar situations. I have used it to fix some overly zealous porting of an aluminum ACVW external filter oil pump cover. Worked well right up until I upgraded to a Gene Berg cast iron cover and I've kept it for some future ACVW engine project that might need one.
Bought some Marinetex to try because it appeared that it could be tinted and have since decided against any of that. Reading it's literature it appeared to be more flexible than I want.
Glass microspheres are easy to find these days, would be a simple way to lighten up any of these compounds (& potentially make them more rigid too) for use in stiffening a polymer stock or in a penultimate ultra-light build where even the mass of the bedding is a concern.