Testing is established. Currently the U. S. government (FBI) tests and evaluates handgun rounds with 'ballistic gelatin'. Which is fairly expensive to buy, but the federal government gets it free via taxes. Available at Midway for about $120.00 per block of 6x6x16 inches. (Little steep for me.)
As an alternate to that, common water is pretty good. However, water has to be moved and contained in some manner to test and evaluate individual shots.
The biggest problem is that of comparability. Ballistic gel (I'm going to use "BG") is supposedly formulated to duplicate actual tissue. It likely does (within tolerances) but water is consistent (like BG). I have a suspicion there is a 'conversion factor' which indicates 10 inches of one is equal (within tolerances) to "X" inches of the other. But how to stack water up to replicate BG? Row of plastic jugs? How much do plastic sides count? Does air space matter?
Another variable is distance from muzzle to test media. No doubt the FBI has a prescribed distance to insure uniformity of test information. But if I think 15 yards is proper and he thinks 2 meters is a good idea we're going to have different results, even in the same media. At one time, I thought two inches of newspaper or old magazines in front of three gallon jugs of water would be useful. This presumes everyone's news paper or magazines, everyone's water and everyone's jugs are pretty much the same. However, that requires a carrier of some sort to hold it all for testing and separately, a means of getting it to the range and back. (For those who can shoot in the back yard, I'm jealous!)
I rather like the set up 'Gun Sam' has on his Youtube channel. Perhaps not perfect, but repeatable.
For transparency, most of my testing is for 1, accuracy and grouping, and 2, muzzle velocity.