1. Obtain 'original' M16A2 body (upper only would be cheaper, full upper and lower Reciever for authenticity)
2. Remove your M4 body (or if you're just changing the Upper, remove the Upper..)
3. Fit M16A2 body (same again depening on circumstance)
4. ???
5. Profit
/thread
okay, you can ignore 4 and 5 if you're not a fan of southpark...

underpants gnomes made me put them in..