What is the little "nub" or protrusion on the rear of the Hi Power barrel above the chamber? As best I can tell, this is similar to the barrel hood on the 1911. Perhaps it was originally expected to help with a tight, consistent lock-up when the slide and barrel are in battery, but the cutout in the breechface is generally pretty loose so I don't think it actually helps in that regard. Some have said that it's for the bullet to strike and bounce downward into the chamber, but I do not believe that. The angle at which the cartridge would be for the bullet to do so would inevitably cause a failure to feed. I've asked two gunsmiths about it and they frankly don't know for sure, either, but both had opinions similar to mine.
Until fairly recently, all Hi Power barrels had these hoods. Current ones that I've seen do not. I own one fairly recent Hi Power that came without it. The barrel locks up just as tight and the gun shoots just as well as those having it. I did notice that the cutout for the older design is still present in the breechface. I don't know if that will continue or not. It may be that FN left it in the process so that older barrels could be used as replacement if necessary, but this is just a guess.
Again, I've noted absolutely no difference in accuracy or reliability in barrels with or without the feature.
This BarSto 9mm Hi Power barrel has the hood, which can be seen at the top of the chamber.
This FN barrel for the Hi Power is a newer one and does not have the hood. Notice also the small cutout on the side. I can only assume that this is intended to allow visual checking to see if a round is chamber without having to retract the slide.