What is the difference in these two cartridges/ They are the sme length, headsize and thickness etc.

Barnes lists the bullet size on a .476 as .472 andthe size ofthe bullet in the .455 as .455.

Is there any simple explanation?

No simple explanation. They are British cartridges. I dug the following out of White & Munhall, Volume II: .455 Eley is a name applied to .455 Revolver Mks I & II. The .455 Revolver cartridge was first made in 1892. There was no ".476 Eley" cartridge.

In the 1870s, the Brits used a "450 Revolver" cartridge for the Adams. Both the revolver & cartridge were less than successful, so in 1882, the Brits replaced them with the Enfield revolver. W&M said the Brits never applied an official caliber designation to this revolver. Inside lubricated cartridges were labeled by the cartridge manufacturer as ".455 Enfield". Cartridges loaded with heel-type bullets were labeled ".476 Enfield". Both were intended for the large Enfield revolver. The Enfield revolver was replaced by the Webley in 1892. Apparently, all of the multitude of .450 & .455 & .476 Revolver cartridges are interchangeable. This was too confusing, even for the Brits, so in 1892, they replaced (or re-labeled) everything as .455 Revolver Mk.I. With a shorter case and smokeless powder, the cartridge became the .455 Revolver Mk.II in 1897.

Whenever anyone attempts anything approaching the encyclopedic and technical, errors will occur. Frank Barnes started out to provide some information on 1500 of the world's more common cartridges. While Barnes kept control, some of the errors were corrected. He never got around to correcting the information on British military revolver cartridges. After Barnes' death, the publisher has used COTW as a cash cow, putting very little effort into correcting known errors, making more errors, and generally letting the quality of the work slide from pretty good to downright sloppy & shoddy.

Richard

This is a very complicated subject so I will stick to the Colt stuff.

The 1873 Single action, the Model 1889 and the Colt New Service were chambered in .476 Enfield. They are marked ".476 Cal.". The grooved diameter is .476", an example cartridge has a bullet diameter of .473" with a hollow base and clay plug.

You cannot chamber a .476 cartridge in a 455 chamber.

It is believed that no more than 15 New Service revolvers were chambered in the .476. This is the only one known at this time.

I have done a bit more digging, both in White & Munhall (1950) and in Erlmeier & Brandt (1980). W&M say that the barrels for "the Enfield revolver", caliber not mentioned, were made at Enfield with the same machinery used to make Martini-Henry barrels. Production of these revolvers was apparently 1880 to 1888. E&B report "There exist 3 versions of the so-called "Enfield Cartridge". .455 Enfield Mark I: The original round, apparently only manufactured in small lots in India in September 1880. .455 Enfield Mark II: Manufacture began in England in November 1880. The bullet was longer than the Mark 1 bullet. .476 Enfield Mark III: Production began in December 1881." The .455 Enfield cartridge had bullets of .450" -.455" diameter, with the idea that the cartridges could be used in the Enfield revolver and also in older revolvers. The diameter of the cartridge case was nominally .477". The length of the loaded cartridge was 1.445" to 1.490". The inside diameter of most Martini-Henry barrels ran about .468". The combination of a .455" bullet fired from a cylinder with a chamber diameter in excess of .477" into a barrel with a diameter of .468" would not be very accurate. The .476 Mk III cartridge had a bullet diameter of .474" to .477", with the hollow base and clay plug. The bullet was a heel design, made to fit into a cartridge case that was nominally .477" in diameter. The loaded .476 Mk. III cartridge was 1.481" to 1.490" long.

W&M report that "Many British Service Revolvers will be found marked on the frame just ahead of the cylinder ".455/.476" This indicates that they will chamber the .455 Enfield, the .476 Enfield, and the later .455 British service cartridges." Apparently, Colt produced revolvers to fit the .476 Enfield Mark III cartridge. It seems logical that the Single Action and the 1889 revolver were produced in small numbers for the .476 cartridge. The .476 cartridge would have been in production, so why turn down sales? But the .476 went out of production in 1892, years before the New Service Colts appeared.

The later .455 British Service Revolver cartridges had cartridge case diameters of .478" or .479", but loaded lengths of 1.429" to 1.448", a bit shorter than the "Enfield cartridges". E&B report the existence of a .476 Enfield Short cartridge, headstamped "KYNOCH 476 C" with a loaded length of 1.264". They say "This was to be fired in .455 caliber revolvers for which the .476 Enfield Mk III was too long". So MP is correct. The .476 may or may not chamber in your .455. It depends on cylinder length.

Thanks to everyone for their imput.

Colt was never consistent on how the marked their guns. They called the .44/40 a cal. 44 while they called its companion a .38 w.c.f. They called the .44 Russian a .44 S & W. They also made .44 Russians to chamber the .44 Special, and .38 Specials that chamberd the .357 Magnum.

I agree the .476 is a very scarce gun.