Howdy
I really, really doubt that your arbor is too long for the hole in the barrel. Do you really think Pietta is shipping these things so that they cannot be assembled correctly? I strongly suspect you are not reassembling it correctly. After all, did you not state that this is your first real gun? Perhaps you are not quite as expert at this as you think?
I suspect the culprit is the wedge. The wedge on these guns can be a bit tricky to get lined up properly, and if it is not lined up properly it can be in the way of the barrel sliding all the way onto the frame.
First, before you do anything else, observe the orientation of the wedge to the frame. Notice which side of the wedge is up, because you may be removing it from the gun. By the way, this little episode should have taught you a very valuable lesson in gun disassembly. Always take note of the relationship of the various parts
before you take them apart. If you had taken a good look at the relationship of the barrel and the frame before you took it apart, you would have known whether or not they were originally assembled properly or not. Now you are guessing. When I used to disassemble a gun that I was not familiar with for the first time I would make sketches showing the relationship of the parts, so I could reassemble them properly. Laying a part on a piece of card and tracing it is a good way to do that, indicating which other parts the part interacts with. Modern digital cameras have made this even easier. Just snap a few photos before you start, and as you go, so you have a record of how things are supposed to go together.
Enough of that lecture. Take your barrel in hand and pull the wedge as far out as it will go without coming out of the frame. The screw on the side of the barrel is there to retain the wedge. The wedge may come out in your hand, that is why you took a good look in the first place to see which side was up, in case it comes out. If the wedge is still retained by the screw, be sure it is completely perpendicular to the barrel. If it is angled, it can present part of its profile as an obstruction to the arbor, and I suspect that is what is causing you trouble. With the wedge withdrawn all the way, and perpendicular to the barrel, the barrel should slide nicely all the way down and contact the frame.
The wedge should look like this, it should be barely retained by the screw head.
With the wedge in this position, peek up inside the arbor hole in the barrel. The wedge should not be obstructing the hole at all. With the wedge in this position you should be able to slide the barrel down onto the arbor. If it does not go quite all the way, give the muzzle a good smack with the palm of your hand. You should not need a hammer, just a good blow with the palm of your hand should be enough to seat the barrel completely down, contacting the frame. Now before you drive the wedge back in, fold a piece of paper over once and insert it between the rear of the barrel and the front of the cylinder. Drive the wedge in until the piece of paper is being gently squeezed between the barrel and the cylinder. Remove the paper, your barrel/cylinder gap has been set the correct amount.
Regarding the timing issues we have been discussing, these are very inexpensive guns. I see Cabellas has them on sale for $199. I think that's about what I paid for my pair a few years ago. No offense, but for $200 you do not get a finely tuned gun. You just don't. That is a bargain basement price. Back in 1860, when the originals were made, there was a great deal of hand fitting done to them to make sure they operated perfectly. And back in 1860 they cost a good deal of money to pay for that hand fitting. There is very little, or none at all hand fitting done on these guns today. They are assembled from bins of parts and the type of hand fitting required to make the bolt pop up perfectly in the correct spot on the cylinder just does not happen. If they were leaving the factory perfectly timed, you would be paying a lot more money. Probably twice as much.
That's why I don't bother to tune mine so they are perfect. I don't shoot them that much, and they are good enough for me with the bolt rising a little bit late.
