I may have stumbled upon part of the reason why.
Big peg, small hole
|From SAAMI (Sporting Arms and Ammunition Manufacturer's Institute). Max bullet diameter listed as 0.355" while Min throat diameter is listed as 0.354"|
Other common rounds, like the .308 Win, have throats that are 0.002" more than the nominal diameter of the projectile. No sane handgun manufacturer would ever choke the gun's ability to feed SAAMI compliant ammo.
Based on that judgement, they reamed the chamber of their universal test barrel with a commercially available ream, one that has a throat diameter of 0.356"-to-0.357".
|Picture from Hornady website. Notice how virtually none of the bullet that is full diameter is in the throat. It is all buried inside the case.|
Other suppliers, like Hornady for instance, take the SAAMI spec at face value. If they are going to make an error, it will be on the side of safety. They custom made a reamer to cut the insanely skinny throat (0.354" dia) and thereby gave up 0.150" of COAL. Otherwise, they cannot get the round to chamber.
That may not sound like much but it noodles out to an additional 0.24cc of volume for the 9mm Parabellum for an increase in powder volume of about 45% over the shorter COAL. The fact that the bullet is immediately swedged down a thousandth of an inch with the SAAMI nominal throat diameter, even as it comes out starting blocks cannot be good for the pressure, either.
Is it too late to petition SAAMI to review the throat diameter they published for the 9mm Parabellum? I think they made a mistake. It should be 0.357" to ensure robust feeding and maximizing powder volume.