MP5 gearbox locked up

[bizzle]

so i traded my m733 for an mp5 and some mags. I test fired at the kids house and it fired fine. i get it home and it fired fine for a few shots than made a sound like the the barrel was blocked. i unjam the barrel and fire a few more, same thing. now the gearbox is totally jammed. pull the trigger and it just clicks, looking into the magwell you can see the air nozzle try to move but its just stuck open. Any ideas on what it could be or can some one cle me in on how to take it apart to get to the gearbox? i pulled the pins in the stock and mag release as well as the selector but i still cant get the lower receiver off. i didnt want to put too much force into trying to get it off 'cause i didnt want to break it more.

help please:confused:

[SACSlym]

Many things could be wrong, you either need to open the gearbox, or take it to a tech.