The 9×19mm Parabellum round is one of the most popular pistol cartridge in the world. Developed as early as 1902 for the Luger P08. After World War I it became popular and many armies and police agencies took pistols and submachine guns chambered with it, in use. Now it is one of the most popular pistol cartridge in the world.
Being used on large numbers of popular pistols and SMGs, the 9×19mm Parabellum is quite easy to find in any gunstore that sells pistols, any police station of countries with armed police, and of course abandoned military bases.
This is very popular cartridge and used by large number of guns. This includes pistols like the M9, Glock 17, 18, 19, 26, 34, many SIG Sauers and more. This also includes submachine guns like the MP5, the UMP9, PP2000, UZI and more. As it is both used by SMGs and pistols you can share ammunition between your primary, if it is an SMG and your secondary, if both use the cartridge. This, alongside the fact that it is not so heavy round, can help you travel light.