[Pistolero]

Yep same barrel.

You probably already know, but since someone out there might not know, here goes.

The 10mm and the .40 caliber are the same diameter.

The question comes into play because it says it's a 38-40 and everybody knows that back in the old days calibers such as 32-20, 44-40, and 38-40 were named that way because the first number stood for the caliber and the second number stood for the number of grains of black powder that pushed the bullet down the barrel, right?

That is true except for the 38-40. The 38-40 is really a .40 caliber bullet, being pushed by 38 grains of black powder.

I am not sure why "they" decided to call it 38-40 instead of what it should have been called, 40-38, but that is the way it is and that is why on that ruger the cylinder is all you have to change.

 

[Pistolero]

If it can shoot 10mm then it should be able to shoot .40S&W, right?

Here is what Ruger says about that;

Notes on the .38-40/10mm Blackhawk Convertible (this model is no longer in production): Ruger New Model Convertible revolvers using the cylinder chambered for the .38-40 cartridge can use all factory loadings of this ammunition both regular and high speed. Do not use any other ammunition in the .38-40 cylinder.

Revolvers using the cylinder chambered for the 10mm pistol cartridge can use all factory loadings of this ammunition both regular and high speed. Do not use any other ammunition in the 10mm cylinder.

The problem is that both the .40S&W and the 10mm are rimless cartridges unlike the rimmed .357mag/.38spl or .44mag/.44spl.

The rimless cartridges in this Ruger revolver get their proper head spacing by how the chamber is bored. Since the .40S&W is shorter than the 10mm, the cartridge would go too far into the cylinder chamber, causing at the least, misfires, or worse, other issues associated with improper headspacing.