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This is another Ruger Blackhawk 'Convertible' Model, this one is in .357 Magnum, and comes with an extra cylinder which chambers 9mm Luger cartridges.
Just like with the other Blackhawk, the 9mm cartridges will chamber and headspace in the chambers, and the ejector rod can poke them out just fine later.
Since .357 Magnum and 9mm Luger differ slightly in caliber (as in projectile size), to get precision with 9mm, this revolver has a slightly tighter than normal bore for .357 Magnum, which will let the 9mm projectiles engage the rifling like normal, but not so tight as to stop .357 Magnum projectiles (or .38 Special projectiles, which are the same), from squeezing through.
This does result in some higher pressure when shooting .357 Magnum and .38 Special, and while this is fine with typical off the shelf amo, this is a reason why this Convertible model isn't as suitable to especially high powered ammunition as the normal Blackhawk, such as Buffalo Bore brand, or certain handloads.
