CYMA MP5 rebuild - what am I missing?

[odobo]

Hi I am building a CYMA MP5 Full Metal upper receiver to be a solid stock version from various parts of previous died MP5's

The problem I currently have is the following:

When connecting the lower receiver to the upper the gear box it fits perfect to the hop unit and the front receiver pin locks perfect and everything looks great!

Right up until the stock (ICS version) slots on the back - it slides on but the pin lock hole is short for where the stock slides to - by about roughly 1 cm so the holes don't line up when they are connected together

So the question really is:

Is this a stock issue - as in the stock isn't sliding on far enough to completely enclose the lower receiver ?

or is it a lower receiver issue - as in the receivers come in different lengths from different manufacturers and its because I just happen to have a short length lower receiver ?

the lower receiver is from a JG mp5 if that helps - any advice would help

thanks