I've tried grey and black, American and British, cheap ($2) and expensive ($6), with and without flat on top, top down and up. The flint strikes the top third of the hammer at about a 10 deg angle and flakes from the edge to the top of the clamp in general. I've tried both leather and lead blankets. It takes a good amount of force to come to full cock. The flints are the recommended size (aprox. 1") I've not tried French flints due to availability. Hope this helps.