OK, checked it out, you can order the Colt Peacemaker in .32/20, .38 Special, .38/40, .357 Mag, .44/40, and .45 Colt.
Not quite Ruger Super Warhawk, but I'm sure Colt will have time to work on the gunmetal enough to build it in whatever caliber the Ruger uses.
Now, if you excuse me, I'm going to rant at the closest brick wall about the lack of calibers listed in Shadowrun. Again.