Poor mans method... dollar store ziplock bags... pour in a known quantity of water into each bag... build a box to hold them .... shoot and see how many bags are damaged. Same as using milk jugs without having to dispose of the milk first. Produce plastic bags would work great and seem to be offered at many locations for "free".... or you can just get a roll from a supplier. This is an ideal way to do a relative comparison... this went through 5 bags... this went through 8. And you can likely find the bullet or fragments.
Rich man would set up a long fish tank, rubber dam at one end, high speed camera (digi cams or even a smart phone can work), "ruler" on the other side and can film the event,
Water is the best material for the average person... and usually readily available. And typically very consistent, heavy metal not withstanding :-0
Jerry
