Ok, I'll just say in 4 + years I've never seen an open top from Uberti, ASM, ASP, Colt 2nd gen., Sig.series or Pietta (pre fix) that didn't have a short arbor. The only open top ever to come to the shop with a correct fit was an 1862 produced '60 Army (the originals . . . ).
You can measure with sticks all day but in the end, a washer (or stack) is the most accurate way of determining the short arbor or, you can just drive the wedge in till it stops and your cylinder will be locked. Either one will prove that your arbor is short. As I "kidded" before, if yours isn't short, it would be the first . . . or an anomaly. You asked "what is that?" about the short arbor. Two of us told you, one gave you links about the problem and how to fix it (there are other ways). It is a known and much discussed topic on many forums. Education is a "good thing" but a waste if it's not accepted.
When it is corrected, you can drive the wedge in and have a predetermined barrel/cylinder clearance (a free turning cyl). I set them to .0025" - .003" . And, they can handle full loads all day, every day . . . from now on !!
Mike