Playing shooters with the Wii is an enormous fun but all the guns I could find were so bad; using the Wii Zapper you need to use a different trigger finger using just the normal Wiimote-mounted pistols is still bad, because using the A-button is complicated...The solution was using an old air-soft MP5 and get a Wiimote in there. Because the first attempt was really a prototype an a friend of me asked me to if I could build him one as well I decided to comment the building to upload it here. It took me about three days to build it and it was a "learning by doing process".

Things you need (if you don't have'm at home) are:
-A MP5 (for this project I bought one if these toyguns on the Internet. They look like a MP5 and when you push the trigger they make and sound and there is an LED lightning inside the muzzle.

-A Wiimote

-Cables, connectors, buttons (I needed 5 of them because the MP5 came with a triggerbutton and two buttons for the foregripp flashlight), soldering-iron and tools like caliper, screwdriver.

-Some plastic plates (I bought them in a local DIY-store)

ATTENTION:
This is what I did with the MP5 I found. If you have a diffrent one it could be possible that you need to make diffrent modifications.
I also want to apologize if my english is too bad, but I life in germany and I'm not a native english speaker! ;-)

Step 1: Taking Appart the Wiimote and the MP5

If you are have a triwing screwdriver use it if not do it the way I did:
get a drill and drill them down until you can open the Wiimote. I'm going to say this only once:
BE CAREFUL!
The Wiimote is a sensitive device. Don't hit the board with the drill and be keep in mind that the screws are going to get hot, so that the plastic may melt, which will make things more complicated.
After opening put all the parts falling apart in a zip-bag, we will need them later.

Same with the MP5: unscrew all screws. I made a little sketch which shows where which screw is because they all had different lengths. There was also a ring on the muzzle I had to cut away and the flashlight, because it was glued. Looks like the boys over there in china wanted to make sure that nobody is copying there stuff. =) Remove all the electric stuff in there but be careful with the trigger unit. I used it instead of constructing a new button there.

Step 2: Soldering on the Wiimote

Because I wanted all the buttons to be used I had do solder cables on the Wiimote board on every button. The only problem is, that the +, -, home, 1 and 2 buttons are small.
Just take the foil over them carefully away and thats it. I used a small cuter.
There is no best place to start with soldering but I thought it wouldn't be bad to start with the small buttons. To find out which side is the right to solder just take a Multimeter, use it in the contactmode (which means it beeps if the two tips have a conductive connection), hold on tip on one channel of the button and the other on another channel on a different button. When the Multimeter beeps it means that those channels are the ones you don't want to solder on. The have the voltage for the buttons on. Solder the cable on the other channel and to that with ever button. All buttons have the same source, so you can keep one tip on this channel and measure which you want to solder on. Don't forget to solder one thicker cable on one of the voltage channels. This will be the "source" cable, whick must go to every button!
I used ductape to keep the cables in place which was to secure them. What also helped me a lot was naming the cables by wrapping around some tape and write on it which button it is. You wanna do that because at the end there are hanging about 15 cables from the Wiimote. It's also helpful to make the cables longer than you can see on my fotos, because of the wiring, which comes later.

Oh, and what you definitively want to do is checking the soldering spots with your Voltmeter. I had a lot of fun taking everything apart because there were some cables not good soldered!!!

Step 3: Push the Button

Now it's time to build "external" cases for all the buttons. The easiest way is to get small buttons from radio shack (or where ever you buy them in the USA), get a small board, put them under the Wii housing, fix them and then start soldering. There are a few pictures which may explain better what I'm talking about.
My decision was to use the control cross on the left side, where the fire-select switch was, the +, - and home buttons should sit on top of the wii and the connect button should go inside the mag bay. Because the B button will be connected to the old trigger button there's no need to build things for it.

We will talk about the A button later.

Step 4: Working on the Hardware

Because the MP5 case wasn't made for the use with a Wiimote some modding had to be done.
I started with sawing away all the rest of the flashlight in the foregripp. Then I looked for the best place for the Wiimote board to bee. Of course somewhere in the foregrip. Down there are some pictures showing where I decided would be the best place. Some inner plastic parts had to be milled away with my Dremel. Again: be careful! It's easy to take the material away but filling holes always looks cheap.
When you're done with this and the wiimote fits in the foregripp think about a way where all the cables can go into the body. I wanted to use pin-connectors, but I seemed to be to complicated so I milled a part in the MP5 body away and some plastic on the foregripp.
The battery case in the grip needed to be changed from 3 cells to 2 I put the connect button

Step 5: Talking About the Wiiboard and Space

The most difficult and trickiest part is getting the Wiimoteboard in the foregripp. I had to remove some of the plastic on the lower side of the foregripp for the nunchuck connector and the capacitor.
First I wanted to desolder the IR-cam but first the solder they used seems to need ultra high heat and I didn't want to destroy the camera! Because the MP5 body had a battery case I cut away the battery clamps on the board.
After the board and all cables fitted in the foregripp there was still some space which I think is good for the A button to go there.
A bit of the board comes out on the front side. I used the lower front side of the Wiimote body, modifies the upper back side to fit on the front and hot glued it on the board.

Step 6: Wiring on the Right Hand Side

The easiest way to avoid cable chaos is to use connectors. I used them to connect one side of the body with the other. Because I milled already on the left side some plastic for the cables from the foregripp to the MP5 body away I used some heat shrink tube on the right hand side of the MP5 body and hotglued the cables to it. on the backend part of the body I used the male connectors and on the other side the females. Because there was a little plastic pin on both sides of the body it was easy to fix them in the right place. On the other body half I connected all the buttons with the cables coming from the Wiimote board. Except the battery cables! I decided to make it easier when adding a charger in later projects I will need them!

Step 7: Re-assembling

This is the last step:
Fix all loose cables inside the body with heat shrink tube and hot glue!
Re-assemble the MP5 body. The best is to start with the connectors, have an eye on the cables inside the body, clip it together until you are at the muzzle and then screw the screws in. Be careful that there are no soldering points on the board in the foregripp get broken.
Then close the foregripp.

Now go to a game dealer you trust, buy The House of the Dead and have fun.

Goodbye Wii Zapper, Welcome WeeP5

The next project is to mount an 2x16 LCD in the MP5 body to provide turobo triggering for each button in choicable times!

If you need help, please feel free to write it in the comments. I'll answer if I know the answer =)