After following the hints in the picture in this topic, I'm stuck. I've cut the main gun in two, filed it down a bit and... the extension doesn't fit

When I say it doesn't fit, I mean it kind of fits. I can get the longer side of the extension partially into the main gun. The stubby side of the extension doesn't fit the rest of the gun, but I'm sure with some filing on the inside of the gun it would (at least partially) fit.
Has anyone here used the extension? Any tips and advice will be much appreciated.
[edit]
After realising that the above isn't all that clear, I've taken some pictures:
Main gun (with most of the gun cut off)
Rest of main gun + extension
Extension