Is the 5.56 and 9mm upper the same, just different ejection port door and deflector? And will colt 9mm parts fit a stripped upper?

a 556 upper receiver can be used for 9mm as long as you have a 9mm  barrel and bolt.

You don't need forward assist with 9mm so many people prefer an upper that doesn't have it.  You can buy the 9mm ejection port kit from Rock River Arms and put it on your 556 receiver.

If you happen to find a dedicated (usually Colt) 9mm receiver, it will not have the gas tube hole and can not be used for a semi auto 556.

The only thing the  upper does is hold the barrel in place, allow for shells to leave the upper, and provide a runway for the bolt to travel.

Only real significant difference between the 2,   556 will have a small hole for gas tube to enter the upper , and have provision for forward assist.

9mm upper will not have a hole for gas tube to enter, and usually have no provision for forward assist,  9mm uppers usually cost a small bit more.

I used a 556 mystery funshow flat top I had laying aroung collecting dust.

See the lack of port door on my upper?  You can either hang the std door assembly --or-- the "9mm" door/deflector assembly.

Will; the forward assist even work with a 9mm bolt? Or would i have to leave the thing out?

The COLT pattern bolt has no notches on the bolt body... the assist is decoration in this case.

I have never installed one to this upper, I do need to fill the hole though, as when I use a suppressor, it is another place for gas to flow out towards my face,  some day I guess I'll do something about it.

Could i mill the bolt to add the forward assist notch, for some odd reason I'd like to have it working.

Well,  I suppose you could, as long as the surface is similar to that of the 556 carrier to where you cut useful notches..  Have to lay mine out tonight and look.  

Be advised as per the fella that cut the ramp on mine, that thing is hard, so carbide cutter probably be needed.  

Might be a cool thing to do, but the 9mm case is so short, so there not likely be any real suface area to restrict chambering, other than a fat round at the case mouth.

i would rather get a slabside upper reciever than try and destroy a bolt.

Were can i get a stripped RRA or Colt 9mm upper.