I have checked on this (and other recent movies) at the IMFDb.org website.
Roland's (highly) customized 1858 revolvers:
_ he carries two in two gun rig
_ the revolvers were built by Lance Peters from scratch based on the Remington 1858 but with larger frames to accomodate double action trigger and swing-out cylinders
_ the revolvers have double action triggers
_ they have add-on knobs on the front of the frames in front of the cylinder but its not clear if these are used as cyclinder latches
_ they have cap'n'ball loading levers (I originally thought they were based on 1875 Remingtons which retained a web under the barrel to allow use in existing 1858 holsters)
_ if the shells look skinny, it's becaause they are .38 Spl
_ the cylinder of one gun swings out to the right, the cylinder of the other swings out to the left
_ Roland carries his guns so the cylinders swing "inward" to facilitate reloading with the off hand
_ he reloads with six round moonclips
_ the ammo loops on Roland's gun belt carry loaded moon clips
They promise the IMFDb page will be updated after the movie is in general release, so the focus now is on Roland's revolvers.
http://www.imfdb.org/wiki/The_Dark_Tower
The carry rig is cool. (One of the guys at the club uses a Remington 1858 in the blackpowder cartridge matches. Course of fire is ten rounds slow fire, ten rounds rapid fire. He brings four cylinders preloaded five rounds each in leather belt pouches.)