I showed a friend of mine whom just received a PHD in Physics some of your testing using water jugs. He asked me if you have ever tried filling the jugs with a non Newtonian fluid (which I had to look up later since I didn't wand him to think I was stupid). He suggested mixing corn starch with water. He seems to think that the the right mix may be able to simulate the resistance of 2 or more jugs depending on mixture ratios. Anyway, thought it would be fun to mention in case you are getting bored.
I am vaguely familiar with that mixture. But I am not sure what the ratio would be to compare it to properly prepared and calibrated ballistics gelatin. Water is a 2 to 1 ratio.
But thanks for the idea.