Sounds to me that your mainspring is way too powerful, and is beating the face of the nipple with each blow. You need to reduce spring tension on that spring, or those springs. If they are V-springs, they usually can be filed down to reduce the tension. If coil springs, then cutting coils will do it. The tension only need to be about 10-15 lbs to ignite the toughest percussion cap. With the gun empty, put the buttt on a bathroom scale, note the weight of the gun on the scale, then slowly cock the hammer back until it clicks in to the Full cock notch. Note the high reading on the scale. Do this 3-5 times and get an average. Subtract the weight of the gun and you know the tension of the spring. Only one arm of the V-spring actually moves when the hammer is cocked, so take a look before removing the spring, and placing it in a vise to begin filing. Always check to see of the spring is rubbing on the lock plate, or anything else is causing the spring to be harder. If you feel the spring is much easier to work when the lock is out of the stock, that is a sure indication that you have wood in the lock mortise rubbing against the spring, that should be removed, so that the spring can work freely, when the lockplate is mounted to the stock.
