Coffinmaker ain't lying. I ain't that difficult to get an arbor bottomed correctly. There's multiple ways to do it...take yer pic.
Weld and finish file the end of the arbor...….drill a hole and screw a brass or steel button to the end......have a new arbor made by a machinist...…use a shim in the bottom of the barrel hole(my favorite)…...buy Pietta and hope fer the best.
I like the shim method because it's cheap and doable by most anyone. Get a little fender washer from the hardware store that is about the right fit in the hole. Get it a little big diameter wise. File till it fits snug enough to stay in the bottom of the hole so it doesn't get lost. Next take a stone and take the shim(fender washer) down to the right size. Use grease or Prursion Blue to put on the end of the arbor and seat the barrel. Then look at the end of the arbor and in the barrel hole. Close is when there's blue on the bottom of the barrel hole and the end of the arbor. Done is when the blue is crushed and moves off the right surfaces(end of the arbor and around the edge of the barrel hole). It depends on how the bottom of the hole is drilled where it can be concave or flat down in there.
The crush fit from the fit being supper tight from the force of the wedge is what will remove all color from the spots where it fits right.
The idea is to get a good bottoming of the arbor simultaneously when the barrels bottom lug is tight to the frame.
Then then you try the cylinder gap visually or with a feeler gauge. You see.....bottoming the arbor doesn't exactly fix-fit the cylinder gap. The gap may be too tight which is the better of the two scenarios of too tight or too loose. You see, bottoming the arbor can lessen a cylinder gap or widen it. The gap you have to begin with may change when the arbor is bottomed nice and tight.
First scenario.....A gap that is too tight merely needs the breech end of the barrel filed or reamed to specification. That's whatever you want from .001 to .006 of an inch or more. Less is better most people will tell you. Some Hombre like wide gaps though.
Second scenario...….A gap that is too wide. That takes a regular bonified gunsmith or a regular bonified Kitchen Table Gunsmith that has experience in that fix. The bottom lug of the barrel needs taken down carefully until the cylinder gap is correct in it's specification. Some might add that they like to take a little off the end of the frame. That means removing the frame locator pins and that gets you in trouble some times. Mess up and a frame is more difficult to replace than a barrel.
Now...… that can be a little tricky but not outside the scope of someone that has confidence in their file use or drill table use or milling machine use. I use the file or the milling machine use to take a little off the barrels bottom lug till the cylinder gap it good to go when the barrel is against the frame and the wedge is set in the barrel. Now...…
If you have a milling machine the cylinder face can be milled down ever so slightly to set the cylinder gap if the gap is too small. There's always some aggravation to this Colt fix it time. It's usually better to find a forum where there are fix it cowboy gunsmiths that can take care of the bottoming of the arbor when the going gets tough. I know.....been the many times. When the job is done and right on it's rewarding. I like that feeling. Then I voluntarily admit myself into an insane asylum for a period of time. Thing is...once they let you in they want to keep you forever
That's is when you fight your way out and then join the Foriegn Legion and hide away for a decade or two.
You see....for instance. When the barrel needs set back to fix a cylinder gap ,and that gets done, there arises another aggravation. What might that be?
Making a "new" wedge to fit the barrel/arbor slot once the barrel is set back and lengthens it is what is needed. That is where you get innovated and make a new wedge with a file or weld the edge of the original wedge and file it nice and fit again or make a new wedge on a milling machine or a good drill table using a mill table that can be put on a drill table. You know...use end mills to mill out a new wedge.
I guess the cylinder gap thing can get tricky but.....the scenario where all is needed is a shim or whatever to fill the gap between the end of the arbor and the bottom of the barrels arbor hole is the most likely one to happen and the easy one to take care. The hardest part of that is to shape the diameter and stone the shim thickness to fit the gap. This is when the cylinder gap is already in specification.
I guess it's all in the luck of the draw when you buy whether it be Uberti or Pietta. I'd guess if the manufacturers were convinced the bottomed arbor is what is right and defected guns without that is what is not right. then people decide it just ain't worth the trouble to buy defected guns so they buy a new bowling ball instead of a gun...….sales would drop off.