Get a letter signed by your father attesting that it belonged to a g-g-g-grandfather. Then contact the National Archives and get his service record. Attaching the story increases the value of the gun. Not that you're going to sell it, but it does make it more interesting when there's a story behind it. You might also see if you can get a regimental history too.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!