Bill,
As long as the old barrels are not cut off, you shouldn't need a fixture. You can get reamers from VME, mentioned in the broken bridle screw thread above. They won't be cheap, but will be much better than trying to use a twist drill. As long as a rifle chamber is not involved (a rifle chamber has too much taper for a reamer to work until it is counterbored large enough to get past the extractor cut then drilled straight. The counterbore is necessary to prevent the drill flutes "grabbing").My friend Walter kept the proper size reamers for 16 ga., right on his lathe, so I didn't have to calculate the correct sizes or the difference in diameters between each step. As I recall, I used about 3 different sizes. You do have to ream past the rim recess. The new tubes were made already turned to size and they set the required diameter of the hole. Since the joint is so large, soft solder is plenty strong enough. I think Walther has a website where you can find the usual standard dimensions of new tubes. This would be useful, even if you turn your own blank from Numrich Arms barrels.
If the barrels have been cut off, the boring bar in a mill is much less likely to get messed up than trying to ream or drill it without a good guide. On the other hand, there is little doubt that barrel sets have been successfully finished working around a good many mistakes. The maker will know the mistakes but others won't. Good luck.
Mike
