I just bought a 6 round gun on the classifieds here, arrived this evening. Annoyingly the action locked up on it as I did some test cycles, I think the internals are going to need some work.
Since I also have a 5 shot ASM, I thought I'd do some quick comparisons to the two cylinders to see how close they are.
Here are the raw measurements:
And here are some quick drawings using the average measurements:
There is a center-line deviation of 2.5 thousands between the two. I'm thinking that isn't nearly enough to matter, and is probably within the margin of error when measuring some old cylinders, one more beat up than the other.
Also, when I installed the 6 shot cylinder on the 5 shot gun, the timing and indexing was perfect. So that's cool, no need for a different hand, everything just works.
I don't know if this info is helpful to you or not, but if you can get someone to measure a Uberti cylinder, you'd be able to see how close they are. At the very least, if you really mess things up, you might be able to find a 5 shot ASM cylinder that would work.