Hello savage458

Interesting thought to make two shoe-lumps together then cut them in half, but no, not the way I used to do them. To cut the barrel beds in my shoe-lumps, I made up a one inch diameter bar 14 inches long, center-drilled on one end, and with a 1/4" square hole through it cross ways at about the 7 inch mark, into which I fitted a cutting tip and a lock screw. This cutting tip is adjustable so the correct radius can be set to suit the barrel diameter. After roughing out the barrel troughs in the mill, I put the bar in the lathe (approx 2" of the bar are put into the 4 jaw chuck and dialed in to as close to zero run-out as possible, and the live center in the center drill hole in the other end of the bar) Then holding the shoe-lump in my milling attachment on my lathe I could true up the troughs. Using the milling attachment I would set the initial convergence into the shoe-lump as well. Still had to smoke in the barrel/trough joint with files etc to make the fit perfect though.
It is basically a horizontal mill/glorified boring bar.

In regard to how much convergence, This is not gospel, it is how I did it, some of my basic findings as follows:
In a 470/500 caliber rifle of an acceptable and reasonable weight, with the centers of the breech around one inch apart, the barrels are set to converge at around 33 feet. With 26” barrels, this put the muzzle centers approx 120-130 thou closer together than the breech. I found for my work this was a very good starting point.
This equates to approximately 5 thou “taper” per inch.

So hypothetically, if the shoe-lump is three inches long, there needs to be 15 thou taper in total. Because there are two barrels, 15 thou divided by two equals 7.5 thou convergence toward center on each side.

The last 500NE double I did had 26" barrels and was 1.025” C to C at the breech and after regulating it, it finished up measuring 0.900” C to C at the muzzles, equaling 125 thou convergence. This rifle weighed at 11-3/4 lbs and it would put 4 shots 2x R/L into 1” at 50 yards.

There are too many variables to try to list and very few absolutes, but from my workshop notes, here’s a quick approximate guide for setting a few different initial barrel convergences, provided the rifle will be an acceptable weight for the chosen caliber.

Smaller calibers:
Working on around 930 to 950 thou between centers at breech and 26” barrels, these calibers need to be about the following measurements closer C to C at the muzzles.
7x57/303 - 35 to 45 thou
300H&H - 60 to 70 thou

Larger calibers:
Working on around one inch between centers at breech and 26” barrels.
9.3x74R/375 - 80 to 90 thou
450 - 100 to 110 thou
470/500 - 120 to 130 thou

I hope this all makes sense.

Best
ALEX