Is this a fair explanation of what you are talking about?
Thanks
Francis
Not really.
A 9mm (or just about any other handgun cartridge that headspaces on the case mouth) has rifling that starts immediately in front of the chamber. The lead into that rifling can be short and abrupt or it can be a little longer and gentle. PLUS, the bore and groove diameters of different barrels can (and often do) vary.
That video addresses free bore in a rifle chamber/barrel setup and that's not quite the issue.
Starting with the bore diameter issue - the bore diameter is the diameter of the bore BEFORE the rifling is cut in the barrel. It is the distance from the top of one land to top of an opposite land. In a 9mm barrel the bore diameter should be about .346".
The groove diameter is the distance from the bottom of one groove to the bottom of an opposite groove. (or if there is a odd number grooves, the diameter of the circle that exactly contains the bottom of all the grooves). On a 9mm barrel that diameter
SHOULD be .355" (it is often larger; with .356" & .357" being common diameters and sometimes even larger).
So, there can be some potential problems with the size of a bullet when dealing with varying types of throats (in this case we are referring to the section of the barrel directly in front of the chamber) AND there can be potential problems with varying bore and groove diameters.
If the rifling starts abruptly a cartridge with a "fat" bullet may not chamber. Even if the casing is the appropriate size for the chamber, a bullet that is too large may prevent the cartridge from fully entering the chamber if the rifling starts abruptly. This can be remedied by "throating" the barrel and making that transition from the chamber to the rifled portion of the barrel a bit more gradual. This provides a bit more room for the bullet when the cartridge is fully seated in the chamber.
Then we have the issues caused by varying bore & groove diameters. Let's say we have a groove diameter of .358" and we use a bullet sized to .356" While that cartridge may chamber without difficulty, that bullet is undersized for the barrel. That combination will likely result in excessive leading due to an incomplete seal between the bullet and the barrel.
SO- we need a bullet that is "fat"
enough to properly seal the barrel BUT
isn't too large to chamber or cause excessive pressure.