There are two different conversations going on here.
First: Chamber size. It's super easy to measure on a revolver. Remove the cylinder from the gun, and use a pair of calipers to take multiple measurements of the chamber mouths. You can get a cheap set of calipers good enough for this kind of work from Harbor Freight. You don't need a set of Mitutoyos.
https://www.harborfreight.com/6-in-composite-digital-caliper-63586.html
Second: What kind of gun is this, original or reproduction. Please take several in-focus photos of the firearm from the top and bottom, showing the entire pistol in the frame. Please include in-focus close-ups of any and all markings.
From what I can see, the finish is
astounding for an original revolver. Even if the gun has been refinished, the stampings, like the name scroll on the top of the barrel, are still super crisp and deep, which would not be so if the gun had been re-polished.
In addition, the frame and loading lever still shows brilliant case hardening colors.
I am not an expert on historical firearms. But this gun is in such immaculate condition that if it
was an original it should not be shot.
I suspect what you have is a second-generation Colt, similar to this one:
https://www.riverjunction.com/3662
The above gun is said to be made in the 1970s, and based on these serial numbers, that would seem correct, as it would for yours:
http://proofhouse.com/colt/1851navy.htm