Some information about rifles, barrels and triggers

Today I saw this post on the frontpage of Imgur

This four barreled hunting rifle might cost an arm and a leg.

As I scrolled through the comment section...

...I saw these two comments and that some people on Imgur are kind of interested in stuff like this. The upper one of @FredianaJimms was not clearly answered, so I decided to take a shot at it (pun intended).
As I got my german hunter's license last year, I had some lessons in weaponry and I constantly try to improve myself in those things I learned. So I'd like to explain to you how these types of rifles work.

Warning! This will be a long text post! Please bear with me, as my grammar is not as well as it could be.

First of all...

...there are many types of combined barrels.
As you can see, the difference is in the type (shotgun barrel, rifle barrel), number of barrels and how they are arranged.
All of these rifles have to be fired somehow. As you know, this is done with triggers. The point is, there are many different systems of triggers, too. I'll show you

My hunting rifle. Calibre .30-06

This is my first rifle. There are many like it, but this one is mine.
I got this one from my dad as a gift.
As you can see, this rifle only has one barrel.

But when you look closer...

...you'll notice two triggers. How does this work, you might ask. I'll explain it to you.
A trigger has a set resistance. You'll have to pull it with the power of a few pound to overcome the resistance. The problem is: The more power you need to pull the trigger, the lower will get your dexterity.
The solution could be: You lower the resistance of the trigger, so you only have to slightly touch it.
This is exactly what the rear trigger does. With this trigger pulled, you'll only need to pull with a power of a few grams, to actually shoot.
In Germany this is called a "Stecher".
There are two types of them: This one here is the "Deutsche Stecher" or "Rückstecher" (further mentioned as "DS")
The second type is the "Französische Stecher" or "Vorstecher" (further mentioned as "FS")
The FS works with with only one trigger, that is pushed to the front, before shooting.

My double barreled shotgun. Calibre 12/70

I assume that most of you have seen something like this before. Simple shotgun, two barrels arranged side by side.

You see, this one has also two triggers

One trigger for each barrel. The front trigger fires the right barrel, the rear trigger fires the left one. Easy right?

This is also a double barreled shotgun

But with a single trigger for both. You pull once, first barrel fired. You pull twice, second barrel fired. With this system you have no choice of which barrel you want to fire first.

Two barreled rifle. Shotgun and rifle combined.

In Germany this is called a "Bockbüchsflinte". Shotgun on top, rifle below. You've noticed, we're coming closer.

Two barrels, two triggers

The front trigger fires the bullet of calibre 7x65. It's equipped with a FS.
The rear trigger is for the shotgun barrel.
Why am I telling you all this, you ask?
So you'll get, what I'm showing you next.

This one is called "Drilling". Translated: Triplet.

It's a combination of a "Bockbüchsflinte" and a double barreled shotgun. This is a common hunting rifle in Germany.
The most common variety is two shotgun barrels on top and one rifle barrel below. But you'll find it with every other combination of barrels on the market.
Notice the poorly circled switch on top of the buttstock. With this thingy, you basically switch the function from shotgun mode to "Bockbüchsflinte" and back again.

Short summary of the function.

Shotgun mode: Front trigger fires right barrel, rear trigger fires left barrel.

"Bockbüchsflinte": Front trigger fires the bullet, rear trigger fires the left shotgun barrel.

The front trigger is also equipped with a FS.
Warning! If the FS is activated, but you pull the rear trigger instead of the trigger on the front, this rifle will fire two barrels at once, because the front trigger will be "pulled" by the recoil.

Finally we have the four barreled rifle.

So, what have we learned before:
There are many possibilities to actually fire a gun.
There are shotguns with only one trigger for two barrels.
There are switches to determinate what a particular trigger has to do.

I searched for long on the internet...

but I haven't found a picture of a four barreled rifle with more than two
triggers. So I assume It works similar to the "Drilling".
Maybe the switch changes the function of both triggers instead of only one.
So it might look like this:
Position 1 -> barrel a and b
Position 2 -> barrel c and d

Unfortunately I never handled a four barreled rifle, so I'm not able to exactly tell you how it works.
All I know is: It definitely does and regarding triggers and shooting bullets nearly anything is possible.

I hope I could teach you a bit in this topic. I definitely learned a bit for myself. If you have questions about this kind of stuff, don't be afraid to ask, I'll try and answer all your questions and maybe make some more posts about this.

Thank you, if you read all this stuff until the end. For those of you, who did: Here's your reward!