Regarding the original question (residue buildup in cylinder holes from shooting shorter 38's in a 357 cylinder), I've found that C3Shooter's method works very well. I generally remove the cylinder, apply bore cleaner liberally, let soak, then stuff a larger brass bore brush (in my case, I use a .44 brush) in the cylinder hole so it pokes out the end little & spin the cylinder around the brush for a few revolutions.
Then I swab the holes out with dry patches, examine the holes & repeat if necessary. I've gotten some pretty dirty used guns clean this way.
Regarding the powder residue on the cylinder face, I've had great luck with Mark F's method...FLITZ baby!
My stainless guns get the Flitz treatment after each outing.
Although, I have yet to find a good product for removing powder residue from the cylinder faces of blued guns...without removing the bluing.