Patrick, some good points made on the shooting of original guns.
I'd like to offer my thoughts on the above mentioned rifle. Due to the configuration of the stock and the style of decoration, I have to say that stylistically the rifle dates to the post-1850 time period and was made as a percussion rifle, it was never a flintlock. The lock can easily date to the mid-percussion period , thus putting the date for the rifle slightly later, my guess would be 1860 - 1870. It was always a percussion lock and is original to the stock inlet. The so called drum and nipple conversion is the simplest way to not only convert a rifle but it is also the easiest way to percussion a new rifle so we find many rifles that were built that way. It has been carefully maintained over the years and the owner should be proud of it as has been suggested.
The hammer appears to be original to the lock, so it is possible that someone over the years unwisely removed the drum (for no good reason, there rarely is) and then, when replacing it, overtightened it throwing out of index. It has been fired many times in it's current position (maybe dry fired?) as the hammer nose is obviously well worn, making actual firing hazardous due to a lack of containment of possible cap fragments.