How do blank cartridges work?
An automatic weapon normally uses the pressure from the firing of a bullet to reload. But what happens in films, in which the cartridges are blank and pressure reduced?
The idea behind an automatic weapon is to use the high pressure caused by the firing of a bullet to reload the weapon. If the weapon fires blank cartridges, the pressure is much lower. So, a blank cartridge amplifier is mounted at the mouth of the barrel. The amplifier reduces the barrel’s circumference, so the recoil is amplified, and the pressure in the barrel becomes high enough to carry the next blank cartridge forward, reloading the weapon. The amplifier is usually red and rather clumsy, as it clearly indicates that the weapon fires blank cartridges.
In films, the same method is used, only the amplifier is hidden inside the barrel. An engineer bores out the barrel, inserting a screw with a 3-5 mm hole. The screw produces a gas pressure that is high enough to reload the weapon, so the shots seem real enough.