steelslaver has posted above the most important contribution to this whole thread. The cheapest way to alter most metal parts is with a file. Ah, but therein lies the rub, acquiring that skill is a task that takes time, concentration, effort, determination and really good files.....No, I mean really good files. A friend of mine went to a gunsmithing school in the early '60s. The first day in the school's shop, all in the class were given a small piece of 1" thick mild steel plate with a 7/8" dia. hole drilled thru the center. Along with that plate they were give a rough saw cut piece of the same steel that was about 1.125"x1.125. They were told that they had all semester to accomplish this task and that they must do it or would have to repeat the semester: File a 1" square opening thru the flat plate that was within + or - 0.005" square; Then file the small piece to pass thru that opening all six ways with no clearance gap being more than 0.001" by feeler gauge. He said that it took him nearly all semester to accomplish his filing job and that some of the others just dropped out of the school.
Now, that is an extreme way to learn to file, but that skill is absolutely invaluable throughout a gunsmith's career. I didn't have to do that particular exercise, but I became very critical of my filing skills after hearing my friend's story and seeing his "Filing Exercise Plate".
An aside: Top quality files make difficult and delicate jobs finish better with less effort. The best files available now are no longer USA made, but IMHO are from the Norse/Denmark area of Europe. They are even better than most German made files. Alas, our old standbys made by Nicholson are now made in China and while they look just like the USA made, the quality is just not equal.
Also; you "eager to learn these skills" people, pay close attention to what steelslaver is posting. He is self taught to a higher degree of skill than most and has the ability to post what he has found to 'work' best.
Also No. 2; Any file job will work best if you have and use Dykem on the surface to be filed and white chalk on the file after a good and frequent cleaning of the file with a wire file brush. Pay particular attention to the use of "Safed" files, protective brass barriers, etc. I have found very thin brass shim stock is particularly useful in preventing collateral damage while filing in tight conditions. I use exactly the same filing method to finish off the barrel/cylinder gap as described by steelslaver. I never saw the need to purchase the mechanical cutting tool he describes to cut that surface. I do have and use the forcing cone cutting tool set from Brownell's as there is just no other way to cut the forcing cone properly and in alignment with the bore of the barrel when the barrel is installed in the frame. ......