The front part of the upper receiver on an MP5 has two little arms that jut out. Those arms are what hold the entire front portion of the gun (including the cocking lever) to the receiver. A bolt goes through the arms, and that holds everything together. The bolt holes cracked through, so the bolt has nowhere to go through. The cocking lever spring travels back into the receiver, basically trying to push the entire cocking assembly (and front of the gun) away from the rest of the gun. So if i play with the cocking lever... I'm sure you know about the MP5 slapping sound when the bolt is released. Basically, if the bolt is allowed to fly forward, the entire front portion of the gun fly's off, leaving just the inner barrel.
I think i'll just wait another year and get a metal receiver. It'll look way better in any case.
Thank you for your help, everyone.