If you have a pre ban upper and you are going to complete it with a post ban lower is it legal to just grind the shoulders off the bayonet lug and leave the lug base.

I hope I have this right.  If you shave off the lug, it will work.  Assuming the barrel is a minimum of 16".  If not, you will have to have a muzzle brake permanently attached to bring it to a min 16" length.

I can't spell anything at this hour.

If the upper was assembled into a complete rifle before the ban I belive that you can attach it to any lower. WAIT FOR ANOTHER ANSWER THOUGH. Im not 100% sure.

For the average citizen, a preban upper [b]cannot[/b] be mated to a post ban lower.  

you got 'upper' and 'lower' mixed up.  what you say is true if it's edited to say this:

You have a few other things to worry about besides the bayo-lug.

1. Is the barrel threaded? If no, not a problem.
  If yes, you must ether cut the barrel to
  remove the threads or permanently attach a
  brake (not a flash suppressor) by either
  welding/blind pinning or whatever method
  ATFE has approved.

2. Is there a flash suppressor installed? If
  Yes, you must remove it and proceed as
  stated in #1.

3. Remove Bayo-lug.

I'm sure this has been addressed before, but how much of the bayo lug must be grinded off?  The whole thing?  Or just enough so that a bayo will no longer attach?

If you cut or grind off the two lttle side lugs right at the front of the piece, it will no longer retain a bayonet, and you are thus legal.  HOWEVER, if you have a barrel with a bayonet lug in the first place, then you almost certainly have threads at the front for a flash suppressor, and that's a whole 'nother issue. Any mzzle attachment that goes on those threads must qualify as a muzzle brake and not a flash suppressor, it must be of a wider diameter than the original supprssor, and it must be permanently attached, either by welding, silver soldering, or pinning with the end of the pin covered w/weld.

the only thing im not sure about to the above thread is IT MUST BE WIDER, dunno bout that..i have seen postban muzzle brakes that were identical to the preban birdcage, jus closed off at the end..looks the same size to me, dunno

They're much much wider at all, and yes, they do look like the issue flash suppressor with the end closed off.  All they have to be is just enough wider so you can't mount a rifle grenade on them.  Have two of them on a couple of my ARs, and unless you put an issue suppressor along side, you can't tell the difference in diameter. [:D]

Your best bet is to remove the origional comp and install a wilson brake then drill and blind pin it in place.

the reason for this is that the outside diamiter of the origional pre ban comp will accept a rifle gernade the wilson brake is larger in outside diamiter and will not accept a rifle gernade.  N.A.T.O. rifle gernades are 20 mm inside  diamiter so any brake that is even slightley larger is acceptable but as previousley posted must be either welded or blind pinned in place. as for the sight base you can keep it however just disabeling the bayonet lug may not be enough for a prosecuter. Remember that the whole ban concept is cosmetic and you still have what resembeles an evil bayonet lug and that could frighten ignorant anti-gun zealots. Just do yourself a favor and grind the whole thing off. Just disabeling it is too much hair splitting for a judge or jury to understand. Unless you have more faith in the average american than I do . Not to mention the cost of an expert to testify in your favor.