Because that's what some more knowledgeable folks than me claim - most of the time the arbors are OK, but sometimes a slightly shorter one is present. I repeat - sometimes. A slightly shorter one. Not like Uberti's every time... I don't claim that it MUST be done, I only suggested to check it and if needed to be corrected. CNC machines are not a guarantee for 100% fail free assembly - I dealt for more than 10 years with Italian construction machinery and I know, first hand, what they can (can't) do.
I absolutely agree that one should check the arbor fit to the barrel lug recess. Mine have never had a problem, even between 3 pistols and exchanging parts. I never said that CNC machined pistols have "fail-free"
assembly. That is up to the factory assemblers. That is far different than having spec parts to assemble.
I don't own a Pietta open top yet, but my observations from recent production guns confirm my suspicions - timing problems, spotty quality, although much, much better then what it was about 10 years ago.
If you don't own one, your "observations" are confined to what you have read online or experienced elsewhere.
Those revolvers are a kit-gun essentially, they must be tuned for reliability and longevity. That's the simple truth - those are some cheap guns, so don't expect Colt like quality and tight tolerances just because they are shiny. And I have no problem with that - I can make it work like it should if needed.
Have you ever assembled and built a repro 1851 Navy
kit gun? I take it you have not. I have and it was very tedious and the results were no where near the same. There are no proof marks, no date code on the frame or other parts, and everything is in the rough. Even the blueing on the barrel and cylinder is just a blush to preclude rust.
As an owner of three Pietta factory assembled models, they are far beyond "kit" quality, sir.
If you do not own a Pietta repro Colt 1851 Navy (which is what this thread is all about), why are you so sure you can fix all its supposed ills? YouTube videos? Other?
Just buy one (a mere pittance) and exercise your skills to your heart's content upon creating the essence of a 1851 Navy repro to excel beyond any of the Colt repros that cost 4-5 times as much.
Or buy your beloved Colt and spend the money.
P.S. My 1858 Pietta, manufactured in 2010 (CF date code) came from the factory with 0.02" barrel to cylinder gap. 0.02" - that's not a typo. We bought two identical guns with the same excessive B/C gaps. So much for the quality of Pietta's CNC machining.
Now you are talking apples and oranges insofar as the difference between open top Colt repros and solid top NMA repros (aka 1858 Remingtons). I have no experience with a repro Remington "1858" NMA so I cannot comment more, other than I am sorry you have a .020" gap on your Remington copy.
Maybe you should have done your homework, sir.
Jim