I have an older Pietta 1860 Army - a late-1980's manufacturing code. Hadn't shot it for years until about a month ago, when I became interested in cap & ball revolvers again. I read about "short arbor syndrome" and tested my Pietta, and sure enough it WAS too short.
My solution was to find a couple of thin washers - actually washers for 1/8" pop-rivets were just about perfect in diameter. I ended up stoning the two washers to get them to just the right thickness (one by itself was too thin, but two together were too thick), then used blue Loc-Tite to "glue" 'em into the bottom of the arbor hole in the barrel.
Fit is perfect now.
Maybe today Pietta fits the arbor correctly, but not back when my old revolver was made.
I understand that the deal was that, back in the day when Colt was making these revolvers, they made all of their arbors too long, and during the barrel fitting process the gunsmith would file the end off of the arbor and keep test fitting and filing until he got it perfect - you can always cut more off, but you can't ever cut more on, so to speak. Pietta (and Uberti) made all of their arbors purposely too short so that any barrel could mate with any arbor/frame.
I understand that Pietta invested in some new, modern CNC machining centers fairly recently. That's probably why the new Piettas have a reputation for their arbors being right - their manufacturing tolerances are so much better with the new machinery.