Its not rocket science and can be done with a drill press and hand tools. But a lathe sure is nice.
You can drill the barrels on a drill press if you have one with a vise and are careful. Clamp the barrel as close to the end the drill will enter as possible.
If doing it this way I would buy a drill smaller than the tap drill size by about 1/16 then buy a chucking reamer about .020 under tap drill sized from MSC. This will give closer to 100% thread depth.
I would buy 2 H1 taps from MSC. Both bottoming. Grind one to cut full depth threads and use the as purchases as a start tap. Use a good tapping oil such as the Castrol Moly bearing tapping oil that MSC sells.
Thread depth should be no more than a little over 5/8 and no less than 1/2".
If you are doing 50 caliber or smaller a 5/8 tap (for 50) or even a 9/16 is OK.
11/16 is good for 54 even with 15/16 barrels. I would not use 3/4 in a 15/16 barrel.
Reaming the hole will give a sharper shoulder at the end of the bore than drilling will but run the reamer very SLOW 100 rpm is plenty fast less OK.
You could have them set up and bored using a lathe and boring bar if you have a friend with a lathe.
This is how I do them and have a 54 barrel in the lathe right now that I will cut off and bore in the AM for a 3/4 x16.
H1 taps are tighter tolerance and will give a better thread fit than the typical H3 or larger taps sold in hardware and auto parts stores. The US made taps from MSC, when available
in the size needed, are also sharper etc in most cases.
I would prefer to make pistol barrels from rifle barrels in most cases, under .50 I am not sure its important. For hunting heavy charges are nice and I don't know how much powder a 22" will shoot in the larger bore sizes like 54-58. The only 54s 22" twist I ever used were in 6" belt pistols and I only used 45 grains in these.
This is a set of 3/4x16 taps. The short one has been in use for quite some time and has had several session of being ground back over the years.
This is tapped deeper than 5/8 but it was a rebarrel and done this way for a reason. But basically the tapped hole should look like this when ready for the plug. Plug should seat at the shoulder.
Dan