Explanation
In all the responses, no one really answered the questions or explained the two-piece barrel.
The barrel of a pistol like the 1911 type or Browning High Power type can be made from a single forging, and the barrels were made that way for years. In recent years, many makers have gone to the cheaper two piece barrel. This allows the back end (the part with the lugs) to be made in one piece then a piece of high quality steel tubing to be rifled, turned down and inserted into a hole drilled in the back end. When properly done and the tube silver soldered or induction brazed, the seam is nearly impossible to detect. But some makers cast the back part, and it is not as strong as a forged component. The barrels do not "come apart", but sometimes the breech splits under pressure even though the inner barrel tube remains intact.
This is simply one more instance where manufacturers try to cut costs to remain competitive, and in so doing make a product that is generally adequate instead of superior.
Air gauging refers to a method of measuring the interior dimensions of a barrel to insure it meets specifications and is uniform. The term itself means nothing in relation to quality. Saying simply that a barrel has been air-gauged is like saying that a frame or slide has been measured. It is implied that the air gauging found the barrel dimensions correct, but that is not necessarily the case. I can air gauge a barrel, find it is way out of spec, and still advertise it as having been air gauged.
Jim