After building my 18 shot Nerf Maverick I have been asked countless times, whether it is possible to build an 18 shot Nerf Strongarm. So I decided to give it a shot (no pun intended :-)).
In comparison a few more alterations are needed and the pieces that connect the barrels are a bit more challenging, but other than that it wasn't overly complicated to build. I still urge you only to attempt this mod, should you be convinced that you are able to execute it. Otherwise you might damage your blaster.
Even if you are not planning on building an 18 shot version of the blaster and are just generally interested in modifying a Strongarm, I think this instructable is worth a read (especially the first step).

Take a look at the video:

Btw. I am completely aware of the irony of taking a blue-orange-white blaster apart in order to sand and prime it and afterwards painting it blue-orange-white again... but as some of you already noticed in the comments, I wanted it to look like something straight out of Borderlands.

Step 1: Common Misconceptions + How to Increase the Speed

I have read multiple times that people suggested to remove the what they call "air-restrictor" from the plunger and to fill the small hole that is on the top (as shown in the third picture).

DON'T DO IT!!!!

Even though they might tell you that this improves the strength of you blaster, it doesn't! In fact, this actually damages your blaster, because it is not an air-restrictor, but an ingenious method to prevent dry firing. Since after building this 18 shot blaster I had some plungers left over I tried to do the mod and to compare the results by measuring the dart speed with a chronometer.
I also removed the barrel posts from one barrel, to measure the effect. Unless you are planning on using Stefans (If you don't know what I am talking about, here is an instructable) I wouldn't suggest you to do so either, because it also decreases the strength of the elite darts.

The only effective way to improve the dart speed that I found was to change the spring! The second picture shows the improvement after adding the Orange Mod Works (OMW) kit (from here).

Step 2: Stuff You Need

3 Nerf Strongarms (e.g. from amazon.com ), or one and two 3D printed barrels
2x15mm M4 screws, M4 washers and M4 cap nut
Jb-weld, plastic weld (e.g. from amazon.com) or some other type of modelling clay.
3D printed parts (you can probably make the pieces from wood, too, but 3D printing will save you a lot of time)
Silicone Grease (not necessary, should you get the "Unleashed Solid Final Stage Kit")
Saw
Strong glue
Sanding Paper

Optional:

Unleashed Solid Final Stage Kit for Nerf Strongarm
Plastic Primer, paint, paint brush

Step 3: Open the Blaster

Start by taking all the screws out of the outer shell. Afterwards take the barrel out. The grey part in the front is loose and can just be pulled off. The one the back is screwed in place, in order to get it off you will first have to pry the two orange shells apart. Start by removing the two screws that are holding them together. You will have to move the grey piece around to reach them. Pulling the pieces apart might take some effort, since they are pressure fitted.

After you have pried the pieces apart, unscrew the small screw in the bottom, to remove the part in the back.

Step 4: Modify the Shell

In order to being able to rotate the barrels use a saw, or a Dremel to remove the rail on the bottom as shown in the pictures. The next thing you will have to do, is to remove some of the plastic from orange front piece, as shown in the second picture. Do so until both sides have the same height.
To make my life easier I decided to design two small plastic pieces, that hold the screw in place. In order for them to fit you will have to remove the small blue rail that is in the way, as shown in the third picture.
As you can see in the fourth picture, I decided to add an extra screw hole, by drilling through an existing hole and filling the end with jb weld. Feel free to do so, too, but it is not really necessary.

Step 5: Filling the Holes

One of the pieces is designed to hold the screw that goes into the front. Glue it in place, as shown in the first picture. Afterwards glue it to the orange piece that goes in the front. It should fit perfectly into the hole.
Glue the pieces that hold the screw in the back in place (as shown in the second picture).

I used jb-weld to fill all the holes, but even though it is extremely stable I would suggest using something with a higher viscosity. The best way I found to prevent the filler from connecting the two shells, is to use tape, as shown in the second picture. Also wrap it around the screw, in order to being able to take the shells apart later.
Once you are done, sand the pieces.

An other thing you will have to do is to shorten the pins that go into the barrels. So that they don't infer with the front of the blaster.

Step 6: Painting

To get the paint to stick perfectly to the blaster, you should sand it. Afterwards prime it and paint it.

Make sure not to paint the bottom of the inside barrel or the inside of the outside piece (click on the second picture to see, what I mean).

Step 7: Assembly

As you can see in the first picture, I added a LED to the side of the blaster, but that is of course completely optional.
Shorten the screws to the correct length. It is easier if you screw the cap nut on, before assembling the barrel. Glue the washer to the connector, as shown in the picture.
Place the connector that goes in the back on top of the barrels, as shown in the picture and add some lube, to reduce the friction. Afterwards reassemble the barrels. Screw the connector that goes in the front in place, as shown in the fourth picture and add some lube to the cylinders.

Step 8: Assembly

The assembly is easier, if you place a book under the blaster, as shown in the last picture. Don't forget to reattach the small spring.
The last picture shows, how the final assembly looks like (click on it to see a bigger image). The main spring can be attached by screwing the old one off and the new one on. Make sure that the plunger is positioned right.

Congratulations, you are done.