Yes, machine guns were used in WW2.
machine guns and rifles
Did you want to know what calibers were used in machine guns or perhaps what cartridges are for machine guns only.
All sides already possessed them before the war.
Efficiency
it used machine guns
Machine guns were first used in the trenches of the First World War. They were mainly used as defensive weapons because they were too heavy to carry, and they required three or so men to operate them. The first true rapid fire gun, the Gatling gun, was used after the Civil War in engagements against Indian tribes.
All of the major players possessed machine guns by the outbreak of the war.
Every country
All of them.
Used extensively.
Tanks were first used in WWI. Zeppellins for observation and bombing. Poison gas as a weapon. There are many other weapons that were widely used in WWI (and are often thought of as debuting there) but were not really used for the first time: Fighter aircraft. Machine guns. Long-range (~100 km) artillery. Hand grenades. Flame throwers. Submarines.