The 9mm is a decently small round, and is also significantly rarer in more northern places of the United States. However, the southern states of Nevada and Arizona do enjoy widespread use of 9mm handguns, 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, JHP (Hand Load) (GRA)
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. However, this round does more damage to the condition of the weapon.
The following is based on the JSawyer mod and has not been confirmed by canon sources.
Junk rounds are makeshift bullets made from materials commonly found while scavenging, such as scrap metal and tin cans. Junk rounds are the only good as a last resort ammo as firing them causes extra wear on the firearm and all around do less damage.
PCXbox 360PlayStation 3 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.
View of the two types of 9mm rounds in-game. Left loaded into 9mm pistols; right loaded into 9mm SMG.