These are types of pistol or two-handed gun with a caliber somewhere around 37mm or 1.5 inches which is designed to fire various projectiles, often used for riot control.

Types of riot gun

• Single-shot, such as the Milkor Stopper 37/38 mm riot gun.
• With a magazine, such as the ARWEN 37 and ARWEN ACE made by the Police Ordnance Company.

Types of projectiles which they fire

• Teargas cartridge.
• Explosive.
• Rubber bullet.
• Incendiary
• Smoke
• GLIMPS (Grenade-Launched Imaging Modular Projectile System). This is a projectile which contains a small camera which transmits TV images of what it sees. See here.