I’m wondering if Beretta might offer the thicker 9mm barrel by itself. I’m also wondering if it is the same OD as the .40 barrel?
However, I have tested a standard 9mm barrel in a .40 slide. Yes, the muzzle end of the barrel was loose in the slide, but that’s also an issue with most 92-style pistols. The main lockup happens in the grooved lug area or the locking block of a 92. Honestly, I’m far from an expert shooter, but in my hands the 9mm barrel in the 40 slide grouped about the same as the 40 barrel.
And since we’re in the subject of PX4 barrels, I just compared a 40 & a 45 barrel. I didn’t put calipers on them, but they appear to be the same OD. The main difference is in the length on the breach side of the lug area.