Complex sentence: They are used to prevent live ammunition from being loaded into a firearm by blocking the chamber with a dummy cartridge or a chamber plug, which is sometimes wedged into place with the use of a tool, in essence jamming the gun.
Split into simpler sentences: They are used to prevent live ammunition from loading into a firearm by blocking the chamber with a dummy (fake) cartridge. The cartridge is sometimes wedged into place with the use of a tool, in essence jamming the gun.