The K'nex M4A1 is easy to built and some of the pieces are bent a bit and it won't ever break. Just get a picture of a M4A1 and try to built it out (thats what i did) Im off a few inches long from the actual M4A1 Carbine.
You can clip in the mag and out but it won't fall out, the aiming is like the one is Call Of Duty 4.
By: Charles . P
Step 1: M4A1 Mag
The mechanism of the mag holder is very simple and it's not that hard to make because they snap together.