[ILLusion]

You'll need to cut down the plastic bracket that secures the mechbox in the stock.

You'll also need to grind down part of the butt plate.

There is also the possibility that you may need to perform some other random grinding inside the stock to take down - areas depending on the size of the cells you get. If you want minimal hassles, get the 1500mAh Ni-Cad cell pack.