I agree on your "Poor Man's Ballistic Gelatin" sentiment.
I've used wetpack, home-made ballistic gel (10% knox gelatin to water by weight, calibrated with a pellet gun), water, and a few other things.
If you just want to test expansion... Save a 6 milkjugs, fill them with water, line them up and aim carefully.
If you want to test penetration too, gelatin is really the only reliable means... You could go to a butcher and ask if they have some pig carcasses or something, but that's a lot of (nasty) work for little reward.
I don't trust wetpack too much after my
Golfball Cannon was able to penetrate two phonebooks.
Try making and calibrating some gelatin, maybe?
Here's a howto:
How to Make Your Own Ballistic Gel
Peace,
Pete Zaria.