Rolf, first off allow me to say welcome to the doublegunshop and secondly congratulations for securing genuine virgin Belgium twist barrels. I looked at the pics on the other message board. (That's an absolutely fascinating reamer set up.) The lump for the lock rail can be left intact and worked to match the locks if you're planning to build a very late flint reproduction. An example of this is the John Furlong 20 bore pictured on page 21 in the book. Otherwise, as I'm sure you already know, it's easily worked away to create an earlier reproduction.
This drawing depicts the basic preparation and joining of two tubes to make a SxS barrel set:
The first step is bench regulating. To bench regulate the barrels you will need a solid, absolutely level and perfectly flat surface to work on. A large table saw or machine tool work bed serves the purpose. You will need three different colored laser pointers, measuring tools; calipers, tape measure and to be very precise, a height gage and two sheets of paper. One sheet is used as the target for the laser and the other is for recording measurements. Make two wooden plugs to match the bore diameter and drill a centered hole in each one to hold a laser pointer. The plugs and pointers go in the muzzle and the third pointer sits between the tubes as a reference point. Set up the target "down range" as far away as possible. Turn the lasers on, lay the barrels on the worktop, clamp them together and put the lasers on the target. Measure the distance between the laser points side to side and from each side to the center reference point. You may notice they have already crossed, that is the left beam is to the right of the center reference point and vice versa.
Now for the tricky part. Grind the flats at the breech taking only a few thousandths of material away, no more than five on the initial pass and one to two on subsequent passes. Repeat the steps above again measuring the point distance side to side. You will notice a big difference at the target and may be amazed by how much difference a few thousandths at the breech makes down range.
I'm sure you're wondering just how deep to go with the flats. It may sound as if I'm dodging the question but it depends. It depends on the barrel wall thickness at the breech. It depends on how far downrange you want the shot columns to converge. Maybe you don't want them to converge at any distance but remain parallel to infinity. As I'm sure you already know most doubles are regulated to converge on a point 30 to 40 yards away. This point can be calculated at the bench using a spreadsheet program. To do this first move the target to within a couple inches away from the muzzle. Measure the distance between the laser points. This distance should be equal to roughly one barrel diameter. Move the target out to five feet or so and measure again. Keep moving the target out an equal distance from the muzzle and measuring the points until you run out of room. This provides a data set to use for extrapolating the point where the shot columns converge. I'll send you a copy my Excel sheet as soon as I find it.
Two minor details that make a big difference: Pay close attention to the distance of each point to the center reference point. If one barrel points inboard more than the other then that barrel will always be off target. If you find this condition exists then grind the offending side a little more to correct it. Be sure to keep both points the same distance side to side from the center reference point. Secondly, pay close attention to the height distance side to side. Generally speaking a well regulated barrel set may have a height difference of two to three inches from the center of the shot column side to side. Any more than that and there will be a lot of missed targets as one barrel may shoot high and the other will shoot low. To correct this condition place shims under the highest barrel at the breech to bring it back in line with the other. This is where the height gage comes in.
Once you're comfortable with how everything looks on the bench the barrels can be joined together. With them tightly clamped together and and everything, including any shims under the breech, clamped securely to the worktop flow in the brass or silver solder. Install the shims between the barrels and weld them in place. Solder the ribs in place, fit the barrels with temporary breech plugs and they're ready to test using a very light charge. Securely clamp the barrels to a picnic table or whatever is available to provide a decent height above the ground and set up the targets. If you find the barrels perform within acceptable limits then give yourself a hearty pat on the back and brag a little -or a lot. If you find otherwise you can still give yourself a pat on the back but the ribs and shims must be to removed and the barrels separated to make the required adjustment. Put everything back together after adjusting and test again. Repeat as required to meet your expectation. Chances are everything will be fine the first time if the grinding, measuring and welding was carefully executed.
One final comment -be cautious and deliberate when grinding the flats. Resist the urge to hurry through the process. Taking too much metal away all at once will lead to unacceptable results. Remove only one or two thousandths at time and sneak up to the goal.
Ok, I've said enough for now.
Hank
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