It is difficult top assess what a designer was thinking unless you can ask him directly I have ran up against this numerous times during my career trying to figure out why someone did what he did when it didn't make any sense to me.
Sometimes there was solid logic, sometimes not.
While playing with my new un-rifled Buck Special A5 barrel that I received yesterday,,, I found the muzzle was Choked to .715 That is -.015 or"Light Modified" choke. Assuming a .730 bore.
One of two things was happening here. Either Browning designed the barrel to shoot Buck Shot exclusively or it deliberately designed the barrel to swage the lands on Rifled Slugs, perhaps to get them to "bite" in the choke and impart spin to the slug? Without being able to talk to an engineer we can only speculate. I might add that the finish on this barrel is a mirror finish, and one of the best I have seen so whatever they did was done for a reason. We just need to figure out what the reason was.
Maybe an Inquiry at
www.trapshooters.com will yield a plausible answer.
Randy
This barrel has marks in the muzzle that indicate slugs have been shot thru it. The Choked area is only about 1" long.