I used nickels because they are larger and heavier, and only made from nickel and copper. I'm not sure about zink alloy in pennies either, but zink is light in weight so pennies will be lighter in mass and have a very thin edge when compared to nickels. If you try pennies, watch out for steel pennies (made during WW2, and rare to find these days, but occasionally may show up if you get a bunch of penny rolls). All that said, nickels work, but not as good as other media mentioned in this thread.
Regarding charcoal for sporting powder, I plan to be making batches of alder, cottonwood, grapevine, and red cedar in the spring. They all grow on my farm. I have very limited amounts of willow by my creek so not worth messing with. I plan to start making my powder with cottonwood charcoal. Is supposed to be very good, and it's way easier to de-bark. If you want to make some good charcoal for sporting powder, get a bag of cedar chips from a local garden center, Tractor Supply Co., or pet shop (not pine...cedar). The chips make nice little coal flakes that are easy to mill on their own. Pine works too, but leaves a lot more fouling in your bore.