It's a real gun, but no antique by a long shot. The engraving may have been done after it was sold. Really need some pics and more info to help you out though. The 1843 date may have been put there to commemorate the date, but the gun itself it 100% NOT from that age.
As far as value from the info you've provided, I'd say between 150-200, but that's a SWAG at best.
What are you gonna do, talk the alien to death? -- (on Sigourney Weaver's worry about Guns in Aliens)
"Safety is something that happens between your ears, not something you hold in your hands."
"I carry a small gun to compensate for my huge Blue press."