This is merely trivial, but Roland's guns, although made in another world, were designed for ammunition that is apparently similar enough to the .45 Winchester that he could re-stock from a modern gun store. However, the .45 Winchester is not meant for revolvers.
It might work if the Excaliburs loaded one cartridge at a time through a small gate, with the cylinder rotated 1/6 of a turn before loading the next. However, even Roland would have extreme difficulty doing this twelve times with two guns while simeltaneously running away from a mob. The Excaliburs would need to be either 'top-break' or have a cylinder which extracts from the gun sideways, like a modern revolver. Both of these designs require a protruding rim around the base of the cartridge, which the Winchester doesn't have.
There are a few possible explanations for this oversight:
1.) King didn't do the research
2.) That world is slightly different to ours
3.) The most interesting explanation, and not suggested by myself: Some of King's fans are rather weird (see 'Misery') and he didn't want any of them copying something in his books, especially if it had to do with guns.
Does anyone else have an opinion on this?