This will be a more detailed look at what happens when a semi-automatic or fully automatic gun is fired. It will also ruin a lot of movies, so there’s that.
First, what is a “blank?”
A blank is a round that lacks a bullet. It’s basically a noisemaker. The pistols fired at track meets use blanks. Some nail guns use blanks. To make a blank, you need to take a normal casing, fill it with powder, then seal the end with a paper disk (revolvers and single shots only) or by crimping the end together to make a point. Some use a mix of the two.
When a blank is fired, the powder burns to create gas. That gas flows down the barrel where it makes a noise as it hits the air.
BLANKS ARE DANGEROUS!
That gas still comes out with enough force to be lethal. If there is anything lodged in the barrel, the blank will fire it like a real bullet. There have been several notable deaths on movie and TV show sets that involved blanks.
Starter pistols don’t have actual barrels. They have a little cone where the barrel should be, so there is no way to do something like put a pencil or BB down it an make a real gun. The gas that comes out the side of the cylinder can still cause damage if it’s close to anything. Most starter pistols are .22 caliber rimfire.
Next, a closer look at actions.
Automatic (semi or fully) actions use a number of methods for cycling the action. The simplest, and the one used by most pistols and sub-machineguns (SMGs) is a blowback or some version of blowback action. Most rifles use a gas trapping system.
Blowback actions rely on Newtonian physics to work. For every action, there is an equal and opposite reaction. When a bullet is fired, the force pushing the bullet down the barrel also pushes the case back into the slide or bolt of the gun. The bullet is lighter than the slide and the force of the spring that holds it closed, so the bullet moves much faster. It’s out of the barrel before the recoil begins (in most cases).
The casing is “blown back” with enough force to push the slide back hard enough to compress the spring all the way, and transfer some of that momentum to the shooter in the form of felt recoil.
Gas Operated systems usually have a bolt that locks the breech closed, rather than a heavy spring that pins it in place. There is still a spring, it’s just not as heavy. When the round is fired, the powder burns and turns into gas. This gas expands rapidly and pushed the bullet down the barrel. This is the case with all bullets, no matter the action. With a gas system, there is a small hole in the barrel near the muzzle that the pressurized gas will enter just before the bullet leaves and the pressure of the gas drops. The still pressurized gas will move along this tube until is hits a plunger. It hit with enough force that it pushes the bolt, unlocking it and driving it backwards far enough to eject the casing and load the next round.
Both of these methods work well, but both rely on there being a bullet to either provide the force to blow the action back, or to keep the gas pressure high enough to enter that gas tube.
With a blank, there is no bullet to push against or trap the gas. Thie means the gun won’t cycle. For a revolver, that doesn’t matter. The hammer coming back is what moves the cylinder. Pump and lever action guns also rely on a physical action from the shooter to cycle. But for an auto, there is nothing for the action to use to function.
To make blanks work for military training, a blank adapter is fixed to the end of the barrel. This is basically like putting a cork in a bottle. The gas can still get out, but the pressure stays high enough to cycle the action. Because there is a physical object bolted to the front of the gun, firing a real bullet will cause damage to the gun, and probably the shooter.
To make blanks work in movie and TV guns the blank adapter has to be in the barrel. This is usually a plug with small holes. There is enough pressure stoppage that both blowback and gas actions will function. Blowback actions can be made more reliable by swapping the recoil springs for a lighter version when using blanks. This is easy enough to do and just takes a few seconds in most cases. For gas systems, the pressure in the barrel is high enough to drive the bolt back normally without any other modifications.
Here's where we ruin movies. There is no way to swap out live rounds for blanks in SMG’s, rifles, or pistols and still have them fire more than one round. There’s no way to put live rounds in a blank gun and not have it blow up the first time it’s fired. I can think of a few beloved, high budget, action movies that do this, but it’s wrong.
The other thing is that the shape of the casings are notably different for blanks, so the ejected rounds, or just seeing the rounds in the magazine will give it away.
For the procedural people… The powder in blank is usually a faster burning than the weapon normally fires. It will leave much more powder residue outside the gun, and it may be chemically different than the powder normally used. You’ll have to research that last bit if it matters. Blanks are way dirtier though, and cleaning the gun after firing blanks very important.
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