This was on a Colt Commander-
First, the hood was a bit too long; the hood would hit the breechface before the top lugs could cleanly lockup. The corners of the lugs were peening a bit. Next, the area below and between the barrel lugs had to be filed and polished to allow the barrel to completely seat in the slide lugs. Brown leaves a little material on the barrel for this purpose. Then the lower link hole had to be slightly elongated to relieve the binding that was caused with the slide stop inserted. And finally, the slide stop pin would strike the radius of the lower lugs while camming up into position. A little shaping was done to let the lugs rotate around the pin and seat n the proper place.
Addmittedly, these were all rather small adjustments, but took some time none the less. I suspect that these barrels are designed to accommodate a wide range of tolerances. I'm actually quite pleased that it didn't just "drop in" as I am now certain that I have a tight, well fitting lockup.
This gun will shoot 1.5" or so at 25 yds. That's more than I require for accuracy.
I'm sure, that many people are able to just install the barrel and fire away. After all, it is advertised as a "drop-in". I wouldn't think twice about it...