Civil War flashback: Thief or spy tricks sentry -- "You didn't keep your powder dry!"

I'm pretty sure this was a feature film. Whatever it is, I'm also fairly certain that it was produced in the last 5 - 6 years.

Someone in the main story, presumably in the 21st century, is telling about something that happened during the American Civil War. Flashback to a soldier standing guard, in the inevitable pouring rain; some guy rolls up on him. Probably the storyteller, if he's a time traveler or very long-lived/immortal. If not, perhaps his great-whatever antecedent, or just someone he heard of. Anyway, the scene keeps cutting between Civil War and present-day. The stranger prompts the sentry to check something on his rifle, maybe the manufacturer's name, which requires him to point the barrel upwards, so rain can fall into it. Some more cutting between timelines, then the stranger snarls, "You didn't keep your powder dry!" and either shoots or overpowers the sentry, who of course can't shoot back.

But that's all I've got! I don't remember the actors, and I don't remember the purpose of this: for spying? To steal gold or something else valuable? Or just to cause mayhem? DH says he vaguely remembers it too, but can't connect it with anything, except that he can't rule out it being something from the MCU. I'm also fairly certain that whatever it is is not 100% real world: either superpowers, time travel, immortality or something else paranormal is involved. Anyone?