Thanks for the suggestion. Others have offered the same idea.
Shooting wet paper is slightly interesting, but with the difference in types of paper, wetness, how tight they are bound, etc, are all variables that will lead to differences in results. There is no way to make all tests exactly the same.
This is a big advantage of ballistic gelatin. If properly mixed and refrigerated, and then calibrated by shooting it with a BB at a specified velocity and measuring the penetration of the BB, it can be confirmed to be exactly the same consistency every time you use it.
But all that mixing, refrigerating, and calibrating is just too much trouble for a poor boy like me. I am not a big money laboratory.
So, I substitute water. Water always has the same consistency and allows comparison from one shoot to another. And the experts have established that water will give a good approximation of penetration into ballistic gelatin at a 2 to 1 ratio. That means penetration of 20 inches of water is just about equal to 10 inches of ballistic gelatin. Bullets will also almost always expand perfectly in water.
Water is fairly cheap ($.62 per gallon at Wallyworld) and easy to clean up. So that's what I use.
But thanks for the suggestion.