while i was at college, i used a sortof indoor ammo
i took 20-25 pennies, then crumbled 1 sheet of notebook paper around them, then took masking tape and taped it all around till it was of proper size( a little bigger than a pingpong ball)
I threw these against my door at college(nomore than 10')
needless to say.. these did not last long...
in the beginning when I really wasnt that strong w/ the sling yet they lasted maybe a week...
i tried right before i left college.... my first throw it broke and rained money in my room
fun but not so good for indoor ammo