Years ago I tried this method of testing bullet penetration and gave up on it. The problem is in most cases it's not very accurate.
For instance, the bullet goes through two jugs and is in the third. So what's the penetration depth?
If there's almost a hole through the back of the third jug you could say the penetration depth is the distance through three jugs, but other than that you have to dismiss the third jug and say the penetration distance is through two jugs.
Another problem is the amount of jugs it takes.
And another problem is I lost a lot of bullets.
The method I finally settled on is more of a comparison between bullets of expansion and a rough guess of penetration.
I use a single one gallon water jug. I've found that if a bullet is going to expand it will do most of it's mushrooming in the first jug.
Behind the jug I have a box of old folded socks so that the capture box has roughly the same density throughout. Fabric stops expanded bullets very well. From the depth the bullets penetrate I can get a VERY ROUGH penetration comparison between bullets.
Also the box captures bullet fragments that are mostly lost when using only water jugs.
If you can set up a video camera sometimes you can later see some interesting results with "freeze frame" that you normally miss.
This is a five gallon jug shot with Aguila IQ 45ACP bullets fired from a Kimber compact at about 10 yards.
The plan was, I was going to shoot the top half of the jug and then immediately shoot the bottom half. It worked perfect.
When the top half was hit the jug rose straight up a couple inches and rolled left, so my second shot caught the jug in mid air and still loaded with a lot of water, which gave two interesting water "explosions".
