It looks like an extremely good gun, and nicely preserved. For many years in breechloaders, people have been proclaiming the advantages of a magnum 12 over a 10, magnum 20 over a 12 and so on. That has gained in truth with shot-cup wads, steel shot etc. But the larger bore and shorter shot column, with less pellets on the outside, is as much of an advantage in a muzzle-loader as ever it was.
You will hear people saying "never shoot an old damascus gun", and it is less strong than drawn or bored steel, but nonetheless a good barrel material if it hasn't been abused. Abuse could include excessive removal of metal to rebrown the outside or remove pitting inside, rust which has penetrated into the welds, or raising dents deeper than should have been raised. But I doubt if that is likely with this gun. It should shoot perfectly well with Pyrodex, but be careful with cleaning, as it can cause corrosion, and is less predictable that way than black. It can lull you into a false sense of security, and then catch you out.
I agree, dirt in the nipple channels could cause the hangfires. More likely than the tight caps I think, since powder might burn in what is effect a powder train, but priming compounds don't do anything slowly. If you need to replace nipples, you can get a better selection on
www.trackofthewolf.com than in the UK, and including oversizes, thread enlargement taps etc.
You would probably enjoy "Shooter's Delight" by Thurlow Craig, my favourite writer on shooting, fishing, animal behaviour and South American revolution, which I first read at the age of eight in the 1950s. He was a pioneer in the renaissance of the muzzle-loader, in the days when it was an even more oddball pursuit than it is today, and there are plenty of copies on
www.bookfinder.com (part of my life support system) and eBay.
I've got mine, but what friends term The Great Scott is a single breech-loader of the 1870s or 80s, and with the need to control recoil with weight of metal, I'd submit it for nitro proof without a qualm.