During WWI, there was a wide range of new and old weapons, including:
Rifles. All nations used more than one type of firearm during the WWI
They had used machine guns.
They had recently developed flamethrowers.
Mortar shells.
Airstrike/artillery.
Poisonous gasses.
Large, steel tanks.
And further developed aircraft.
Have a great day!
-Pepetreefrogthe2nd