That is true, but if you use potting soil as I did and a 10lb tamper your medium is pretty consistent, however the measurement into it would be the issue, most of my shots shot thru a 5 gallon bucket turned on it side with packed top soild, you could use two of them, but it is a lot of work to pick up the loose soil and retamp it into the 5 gallon bucket after each shot. However that is the only measure of "penetration", is something into some consistent medium, such as soil, sand or clay. Chap