Yeah but if you add a tightbore and a better hop-up unit to an M16

I dunno, before you buy an M16, hold a meter stick like a rifle, walk around your house a bit. It'll look really weird, but you would get a sense of what it's like, if my memory serves me correctly, an M16 is slightly longer than 1m.
You don't really need any upgrades for a while. Don't throw away good parts.