The age of this gun I would say is 1830-40 although the barrel is much early ,has I said this type of lock was carried through from the flintlock period with the same internal workings and never changes for a century or more .
Has for the barrel it is 36 inchs long , made of iron and is a 16 gauge,it is a Madrid barrel see image with a crucifix,
the shield would be the maker name, and the other would be his mark , although these have now disapeared i have enclosed a image of a simular gun that bears these marks.
Feltwad