I have been lurking around for several months.  I have spent the bulk of my time here in the tech forum and have learned more about the AR than I will probably ever use.  This site is great.  Having said that I have a question that I haven't seen posted, doesn't mean it hasn't been, just I haven't seen it.  I have a Colt MT6700 post ban AR.   It is HBAR Match Grade Competition.  I love the way it shoots.  It is extremely accurate and loads of fun to shoot.  My question is this, I would like to build a 16" upper geared more toward lightweight easier to handle.  Will basically any components work with the stock post ban Colt lower or are there differences that will make this overly complicated.  I realize I have the option of building or buying a complete rifle.  Just looking for info.  Thanks in advance and thanks for the great site.

Simply purchase the upper of your choice, push out the two take down pins on your rifle, remove Colt upper.  Set new upper into place and push the pins back in.  That should be all it takes unless you are going to use the same charging handle and bolt assembly in both rifles.  They will swap out also but I am of the opinion that the bolt wears to fit and it is better to keep a bolt with it's barrel.  I have swapped bolts with no problems.
Not trying to make this a joke. There can be some differences in older Colt trigger and hammer pins (think they are all mill. spec now) and of course the old large hole and small hole(standard size now) lowers but if all you want is a new upper, there should be no problems.  If you were trying to change some of the lower parts there could be more information needed.
I personally purchase the new upper with charger handle, bolt, and carrier assembly.  That way you only need the lower assembly when you build your next rifle......always happens.  Should say that any brand should fit and function fine.  All parts manufactuerd today hold pretty good tolerances.

Quoted: Simply purchase the upper of your choice, push out the two take down pins on your rifle, remove Colt upper.  Set new upper into place and push the pins back in.  That should be all it takes unless you are going to use the same charging handle and bolt assembly in both rifles.  They will swap out also but I am of the opinion that the bolt wears to fit and it is better to keep a bolt with it's barrel.  I have swapped bolts with no problems. Not trying to make this a joke. There can be some differences in older Colt trigger and hammer pins (think they are all mill. spec now) and of course the old large hole and small hole(standard size now) lowers but if all you want is a new upper, there should be no problems.  If you were trying to change some of the lower parts there could be more information needed. I personally purchase the new upper with charger handle, bolt, and carrier assembly.  That way you only need the lower assembly when you build your next rifle......always happens.  Should say that any brand should fit and function fine.  All parts manufactuerd today hold pretty good tolerances.

Thanks for the info.  I appreciate the quick response.  

If you use an upper that is not Colt you will probably need a conversion pin for the front push pin. The Colt pin is larger in diameter than the standard spec. Otherwise everything is the same. The only other difference is, as mentioned, the trigger and hammer pins on the recent Colts are a little larger than normal. Stocks and other accesories are interchangable.

The Colt he has does have a lower that has the mil-spec small hole front pivot. It just comes with the Colt style two piece pivot pin instead of the military style one piece push pin. As for changes just add a Colt 16inch M4 carbine barrel or Colt 16inch HBAR barrel and a Colt collapsible stock and you will have what your looking for.