Looking at the photo , the hammer isn't striking squarely on the nipple due to the drum being slightly rotated clockwise , beyond where the nipple should be pointing to the center of the hammer nose. This can easily be corrected by removing the drum , and installing a shall piece of say , .007 thick shim stock cut into a washer fitting under the drum. This will cause the drum to stop directly under the hammer nose. This over rotation of a percussion drum , these days , is caused by unknowing persons unnecessarily , removing the drum to clean the rifle . Most rifle barrels are made of soft iron , or today , soft lead bearing steel that can't take repeated tightening of a drum , and threads are stretched in barrel steel. I've had to put shim stock under a couple rifle drums to fix this problem. If done neatly , the shim stock is not visible. To safely drill the drum thread hole in sheet metal , clamp it between two PCs. of wood , them using metal shears , cut the excess away.