I was perusing GunBroker one day after reading up on the Holmes .50 cal, and came across a surplus M2 barrel. The Holmes .50 build plans suggest starting off with a simple barrel blank from any barrel manufacturer, but mentions that a surplus barrel from a M2 might be the easiest path to get started on. The surplus M2 barrels are already chambered and threaded for attaching to a receiver. I thought this to be worthwhile advice, and without any other further issue I bought the surplus barrel off of GunBroker for a couple hundred bucks.
A few days later, the barrel showed up in the mail. A quick measurement or two and it was off to the bandsaw to trim the barrel down to a size I thought would look at least somewhat proportional. I had decided early on that I didn't want to build the receiver as long as was suggested in the Holmes plans, so I wanted the barrel to look correct in relation to the receiver. So, I sawed the barrel down to about 23 inches and chucked it up in the lathe.
I turned the outside of the barrel down in sections, due to it having a really odd profile from the factory. The Holmes plans suggest a 1.75 major OD for the part of the barrel that fits inside the receiver, and then a 1" major OD for the visible part of the barrel. I couldn't achieve the 1.75" dimension with the surplus barrel I had, so I turned the initial diameter down to 1.5" and then turned a small shoulder halfway down the barrel to 1.375" for the barrel retaining nut.
I did stick to the 1" diameter for the remainder of the barrel. In retrospect I wish I hadn't done this, I kinda wanted to flute the barrel but there really isn't enough meat to the barrel to do this. I turned a small shoulder on the muzzle of the barrel and prepared the barrel for threading by machining a tiny taper on the end of each part to be threaded. I have yet to crown the muzzle, but am waiting to do that until a later date.
After I got the barrel turned to the diameter I wanted, I turned a 4 degree taper on the portion of the barrel directly above the chamber. The barrel was 1.7" at this point only, so I turned it down to the 4 degree taper over a ~2" length. Once the taper was finished, I tried to set my lathe up to turn some threads only to find that whoever had cobbled my lathe together had made some really odd modifications to the change gear set up. I tried to get the gears setup for threading 12 tpi, but wound up breaking the bushing assembly on the threading assembly.
A few weeks later, I had the threading setup back up and running on my lathe, and got around to threading the barrel, finally. The chamber of the barrel comes threaded from the factory, but it is not threaded even close to what Holmes suggests. Holmes suggests a 1.5" diameter threaded to 12 tpi, but the surplus barrel came with a 1.44" major OD 1.3" minor od threaded 8 tpi. This was only mildly problematic, but I got past it. Anyway, I finally got the muzzle and the barrel nut area threaded.
When all this was done to the barrel and I still had the threading gears set up on the lathe, I decided to machine and thread the barrel retaining nut. I had purchased a good 2 foot section of 4140 steel round 2" in diameter and a 6" section of 4140 round 2.25 diameter. The barrel nut was machined from the 2.25 diameter round stock and parted off on the lathe. For those of you who haven't ever done it, I highly suggest a carbide parting-off tool for working with 4140. I used a HSS blank and honed it to the correct shape, but still had to engage the lathe back gears in order to keep from melting through the cutting bit. 4140 is pretty freaking hard stuff.
With the barrel retaining nut machined and threaded, I knurled the outside diameter so I could get a grip on it for tightening down the barrel.