It sounds as though the existing bushing you used to make a replacement may also be an undersized and incorrect replacement. Measuring female threads is never as easy as male threads. I have had some success by making an undersized slightly tapered dowel out of soft wood like balsa or pine, and then wrapping it counter-clockwise with several layers of aluminum foil. For small threaded holes, you can sand down a wooden match stick to make the tapered dowel. Then screw the foil wrapped tapered dowel into the female threads until it is snug enough to get a good thread impression. Then carefully unscrew it and try to get an accurate measurement of the impression with thread gauges, calipers, etc. At one point in time, I had daily access to an optical comparator which made this kind of size determination much easier. Many well equipped machine shops have one, and a comparator may be the most accurate means of determining exactly what is the thread type, diameter, pitch, etc., short of getting the information from a factory blueprint.
I have also heard of using Cerrosafe to pour a cast of internal threads, which can then be removed and measured. Of course, you would have to use clay or something to dam the open backside, and also pour extra into a nut or something on the breech face to have a means to remove the cast. Cerrosafe shrinks a small amount several minutes after casting, but I'd probably want to lightly coat the threads and breech face with a little graphite lube to ensure easy removal.
I don't know why Beretta would stamp a gun as 28 ga., but have the bores .019" under nominal bore size. But it seems better than finding out a gun is oversized because it has been honed to death.