I just purchased a pre-ban Colt SP1.  I would like to take a spare 16" flattop upper and convert it to pre-ban to use on this gun.  Really, the only thing I want to do to this upper is add a 3 prong flash hidder.

The barrel diameter on the crown end is approximatly 11/16" which runs 3/4" toward the receiver and then the barrel becomes about 2/16" larger.  Can you get a flash suppressor that will fit on this barrel without having to have the barrel milled clear down to 1/2?  What is the normal procedure for doing this, eg adding a flash hider on this size of barrel?

Thanks

Either buy a threaded barrel, or have yours threaded.

It is not milled.  It is turned down in a lathe.  Only the end tip is turned down and threaded.  All M16 flash hiders have the same 28-1/2" threading.