Many years ago, I left the percussion caps at home and could only find similar toy caps. My experience was similar, about 85% in my Ruger old army. Problem was that if the hammer hit too hard, the circcular nipple cut through the plastic leaving a plastic disc in the nipple opening. The success on my cheap as crap FIE revolver with the nipples hammered flat with a small center hole, by the previous owner shooting the empty gun at TV bad guys, left a more suitable surface for the plastic caps and they worked better, perhaps about 95%