[HAMR]

I have a M4 cqb s but I'd like to adopt a lmg support roll
- - NERD ALERT- -
Im a fan of warhammer 40k and wanted to build a body kit for the heavy bolter and use the AEG as the powersorce. Essentially I'll be building a shell and AEG will go in a idea.
The shell is so that there is no permanent modification to the AEG.

Is this allowed or am I just crazy?

Also. How do you exchange the barrel on an AEG to a longer one.