Pete,

I used to get bundles of newspapers that were going to be recycled. Eventually they were, missing the material from the holes. I shot them dry, because a thoroughly wetted bundle is just a mess, and a solid block of ice if you want to do testing in the winter. Its a little more abusive to the bullets, but that would do more to expand/deform the PRB.

Cow heads are also a good medium, but that's another story.

OldBob