Defacing/destroying US currency is only illegal if you have "fraudulent intent".
When I was a banker, I'd have people all the time walking up to the counter with 20s and say "Can you break that for me?"
For my more known, regulars, I'd say "Sure!", tear it in half and hand it back. The looks were priceless!
Once had a guy about 2 back in line flip out when he saw it.
he told me he just witnesses a felony and was calling the FBI.
Anyway, zinc makes bad bullets, legal or otherwise. Not to mention, you think 1c each penny is cheap... imagine how many pennies it would take to make a typical bullet. You'd be better off, by far I imagine, to buy regular, jacketed bullets.