Shotguns rounds differ from handgun and rifle rounds in three main ways.
The first is that they are mostly plastic. Some are actually paper, but they can’t be reloaded. More accurately, they shouldn’t be. Shotgun casings are also commonly referred to as Hulls, and can come in a number of colors. Most 12 gauge hulls are red. Most 20 gauge are yellow, and most 10 gauge are black. 16 and 28 gauge are rare enough now that I can’t recall which is purple, and have no idea what the other one is.
You can’t just go by color though. Many 12 gauge hulls come in black, green, and blue as well. At one time, these colors may have meant something specific, but after the first firing, they can all be reloaded and whatever that distinction may have been is lost.
The second thing is the “bullet” isn’t one mass of lead in most cases. It’s actually made up of several to hundreds of smaller balls, collectively called Shot, or Pellets. The larger the physical diameter of the shot, the smaller the number. Number 8 shot will be smaller than number 7. 0-buck, or “ought buck” will be rather large. Triple-ought (000) pellets are a little bigger than a 9mm bullet.
Finally, there is the Wad, or Shot Cup. This is a plastic thing that sits between the shot and the powder. It some ways it acts like a little shock absorber, crushing slightly as the powder first starts to expand and reducing the acceleration curve to smooth it out a bit. That’s the idea anyway. It’s main purpose is to keep the shot in front of the powder so it does what it’s supposed to do.
Now, the process.
Sizing, de-priming and adding powder are the same as with handgun and rifle rounds, but then you need to add the wad. Here is where your Van Helsing homage slips the casing out of the press and adds whatever he wants to shoot that isn’t the lead or steel pellets. It could be toothpicks, or diamonds, or a bullet that’s already been fired by someone else’s gun then wrapped in toilet paper which will come off when the bullet hits the air at 1400 feet per second. Whatever it is, this is when you’d do it.
The final two stations put the pre-crimp and crimp on the shell. Because shotgun shells are made of plastic, the ends are folded over to seal the shot in. The crimp ensures that this fold is done properly.
Now, not all shotgun loads fire shot. They also mage slugs. This is a 1 to 1.25 ounce (28 to 35 gram) lead bullet about 0.7 inches (18mm) in diameter. That’s almost ¾ of an inch, or close to the size of the last joint in your thumb. This is for a 12 gauge.
Sometimes these slugs are specially made to include rifling on the slug itself to stabilize it in flight. Other times it’s just a solid, round ball. Slugs can be accurate and quite lethal out to 200 yard/meters or more.
Other specialty rounds include small firecracker type rounds that can be used to scare birds out of fields. These work sort of like grenade launchers in that, you fire them in a high arc and wait for them to explode in the air, or after they land on the ground.
A more lethal special round is called Dragon Fire, and actually turns a shotgun into a shot lived flamethrower. Obviously, this is both very hard on the shotgun, and amazingly hard for the average person to get their hands on. A clever fictional character might be able to make some in a basement though…
Parachute flares are also possible.
Whatever your characters plan to load into the shotgun shell, the reloading phase is where it will happen.