Just bumping this topic up, as people need to know that there are other fantastic ammo options for all types of shooting out there besides the norm of steel balls, lead, marbles, and clay. Just for some examples, there are hex nuts, castle nuts, acorn nuts like those above, tight balls of metal wire, cut re-bar, and basically anything quite small and cube/spherical/conical shaped with some weight. The heavier the irregular object the straighter it will fly despite it's irregularities.....
I use acorn nuts as much as I can, and I always have the eye out for them....I almost never spend money on steel balls and I try not to spend money on any other ammo, so I gather stones, buy a bag of marbles here and there, and scrounge loads of hex nuts.
Good post Flip, they will certainly do a number on any squirrel caught munching on an acorn for too long nearby. Hex nuts will as well, in fact my first kill was with one.
Cheers - John