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What markings do you see on that gun (patent dates, words, numbers, etc.)?
That looks to be a sporting rifle or a "plains rifle." Especially with that set trigger, minimal projecting spurs, and half-stock. They were made in the early 1800s through 1870. While that rifle certainly wouldn't have been issued by the U.S. military for the Spanish American war, which started in 1898, it might have been possible for someone to bring it as a private weapon.
Plains rifles were originally designed for use in the western frontier. They caught on like wildfire and were handmade by wide variety of gunsmiths. The lack of markings on that gun makes determining its background and maker extremely difficult. Without it being made by a well-known maker, such as Hawken, the value is most likely going to be around $1,000-$3,000 or so if in working order.
You could take it to a local auction house that specializes in antique firearms to see if they can find something to hint at that gun's provenance.
I hope this helps.