There should be no problem with the pellets passing through the cardboard. An advantage of shooting into the putty is that it doesn't create any lead dust. If you have decent ventilation or don't mind cleaning up dust(and inhaling a little) alot shooting into a steel plate should work fine.
If you haven't already, check out Target Talk: http://www.targettalk.org/
If you haven't already, check out Target Talk: http://www.targettalk.org/
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