This instructable shows you how to fix the Stinger R34, although since the concept is generally similar for most spring-powered airsoft guns, you should be able to use this for other guns.

NOTE: Before doing this instructable make sure that the gun is discharged in a safe direction.

UPDATE: I added Step 12 since the super glue didn't do the trick. I recommend you do all the steps for maximum strength.

Step 1: The Tools

You don't need much.

Super Glue --Highly reccomended. Holds great.
Medium screwdriver--For removing screws around the rails
Small screwdriver --For removing screws on the side
Paper clip --For reinforcement, not shown in this instructable
Drill bit --For reinforcement, not shown in this instructable

Step 2: Take the Accessories Off

This is easy
Take off the clip (push the button and pull)
Take off the stock (pull the button and lift)
Unscrew any accessories on the rails in front and take off.

Step 3: Take Off the Rails

The steps to do this are as follows:
1: Pull the ring back
2: Pull off bottom
3: Pull off top
4: Slide ring and spring foreward and unscrew piece shown in 3:

Step 4: Unscrew the Side of the Gun.

There are a lot of screws
You'll need to unscrew them all and put them someplace where you won't lost them or knock them off the table.

Step 5: Pull Off the Side of the Gun

You'll need to lift the front end first because otherwise the cocking arm will catch the side and cause more problems than you really want.

Step 6: Glue All Pieces Together/Back in Place

Glue any pieces that snapped off the gun into the proper place. If a piece is broken in two spots, glue it together before putting it into the gun.
In pic. 1 the piece just had to be glued into the right place.
In pic. 2 the 2 pieces had to be glued together then glued into place.
pic. 3 shows how the gun is properly assembled.

Step 7: Put the Side Back On

Just do this the same way you took it off. Make sure it is nice and flush in all areas then screw all the screws that look like this onto the side

Step 8: Put the Spring and Ring Back On

Screw on the small ring with the two short screws. The black barrel will need to be in the proper place or the sides will not close for you.

Step 9: Replace the Rails

Simply pull back on the ring and pop the rails back into place.

Step 10: UH-OH THERE'S STILL SOME PIECES

Wondering where these pices go??
They go right by the clip.

Fit the L shaped one into the slot that is shaped like it and push the cylindrical shaped part through to the other side.

Put the spring around the cylindrical shaped part protruding from the other side

Screw the oval-shaped piece onto the cylindrical shaped part.

Step 11: You're Done!!

FIRE AWAY!!!

But make sure you wear proper eye protection so that you don't get blinded by a ricochet.

Step 12: The XTRA Reinforcement

OK...
The super glue held for a couple games of airsofting, but then it broke again and left me in exactly the same spot. So... Here's what I did: (sorry, but i didn't take any pics)

Materiels for this step:
-Drill (I used a drill press but a handheld might work)
-Paper Clip (the bigger the better)
-Small drill bit (same size as paper clip)

1: Drill a hole at the pivot point for the broken part (This is when the gun is closed)

2: Stick a paper clip into the hole

3: Cut off the paper clip with some sticking out of each side then bend the ends over.