Other than the contradictions in the books over his guns being single or double-action, there also seems to be confusion over the cylinders.
When King talks about Roland's reloading technique in The Gunslinger, you get the impression that the revolvers break or open on a hinge as some old west era pistols did so he could simply drop six rounds in at once.
However, many illustrations, in the books and graphic novels, show the guns clearly as models that load and empty one shell at a time through a port behind the cylinder--very common for old west guns, especially for black power guns that were converted for use with modern shells.
This doesn't lend itself well to a fast reload of any kind.
And, yet again, in Wizard and Glass there is a reference to Roland swinging out the cylinder of his gun, as if it were a modern wheel gun. So...in short, there are no guns that could match all the descriptions of the Big Irons.