The 9mm is a medium caliber auto-pistol round, suitable for use in handguns and submachine guns. While significantly rarer in more northern places of the United States, the southern states of Nevada and Arizona do enjoy widespread use of 9mm chambered weapons, compared to the more powerful, popular, but expensive 10mm firearms.
Hollow point bullets mushroom and break up on impact, causing massive trauma on fleshy, unarmored targets. However, this also means they break up when they hit things like armor plate, drastically reducing the weapon's penetrating power.
9mm round, +P
Designed to fire at a higher internal pressure than standard ammunition to ensure higher velocity, stopping power and a slight armor piercing ability but at the cost of requiring more maintenance than usual. +P Ammunition can be bought at most vendors in small quantities, but can be purchased in mass quantities from Bardon at Hoover Dam.
9mm round, JHP hand load
A new ammunition added with the add-on Gun Runners' Arsenal, JHP, or jacketed hollow point, uses a metal-clad bullet which retains most of the damage bonus of ordinary hollow point rounds, but with a less severe armor penalty.
pcxbox360ps3 Although +P rounds are available from most vendors, their scarcity from some vendors, particularly Gun Runners, appears to be unintentional. Specifically, +P rounds are not included in a vendor list which contains all other non-standard types of bullet ammunition.
Behind the scenes
The 9mm ammunition is comparable to the real world 9x19mm Luger/Parabellum/NATO.