P.S. I don't really know why I'm posting this seeing how it can be built from one picture for the most part =P
Step 1: Handle/ Magazine
1-3. All very easy. Make the first two and attach like so.
That wasn't so bad huh? Moving on.
Step 2: Top of the gun
1. This is optional but I would add this white rod to prevent the back side of the magazine front bending while cocking.
2. Gather the pieces and build what is seen
3. Attach to the magazine by green rods...
4. Then add the white rods.
Alright still pretty easy? Let's continue.