Dan brought his tools to one of our guild meetings several years ago. I was looking through the Brownell's catalog at some point after that and had a "light bulb" moment. There are some commercially available tools that can be modified to replicate what Dan has done here.
I bought one tool that is used for crowning the muzzle of barrels. It screws into a small aluminum handle. Brass pilots are available that have a shaft that fits into a hole in the front of the tool. The pilot shaft is held in place with a set screw. Here are a couple of photos.
I also bought a tool that's designed to face the inside surface of revolver barrels. In normal use... a rod is inserted into the revolver from the muzzle. The facing tool is then screwed onto the rod. The rod is pulled so the tool touches the barrel extension inside the frame opening. It's then turned to remove material from that end of the barrel. Here is a photo of 2 of these tools in different sizes. One size fits nicely inside a 5/8" threaded barrel breech... the other fits a 3/4" breech. This photo also shows the brass pilot mentioned above.
You'll notice that the cutting face of the tool looks like the one Dan made. So... here's how I modified them...
I first took one of the handles for the muzzle tool and filed a slot into one face. There's actually a slot on each side as well, but that wasn't really needed. Anyway... I cut out some steel to fit the slot. In my case, the steel "blade" is "C" shaped, but it didn't need to be. A simple square piece would have been fine. I epoxied this steel "blade" into the slot in the handle. This blade fits closely into the slot that's already in the back end of the facing tools. Basically what I did was turn one of these handles into a screwdriver to turn the facing tool.
The next thing I did was take some extra pilots in different sizes and chucked each one into a small lathe. A hole was drilled through each one to fit a socket head cap screw of the same thread as the hole in the front of the facing tools. Since the pilots are made in cartridge gun sizes I had to turn each one down to fit our muzzle loader sizes. I made then in .36, .45, .50,. and .54 caliber.
Here's a photo of the modified facing tool parts disassembled...
And assembled...
To face the shoulder inside the breech end of a muzzle loader barrel I simply insert the tool into the breech end and turn it by hand. The tool diameter being a close fit, along with the pilot fitting inside the bore, ensures some level of precision. It cuts so well by hand... turning it with a power tool would be dangerous. Just a couple of turns is all it takes to clean up the shoulder. Keep in mind that your factory fitted breech plug won't fit anymore. You'll need to properly fit and index the plug... either by using a new (longer) plug... or by removing some metal off the end of the barrel.