Is there any difference between a FA M-16`s upper and a SA AR-15`s upper? Tchnically and legaly speaking.

Technically (& legally), the uppers are the same, just the bolt carrier is different from M16 to AR15.  Legally speaking, do NOT find yourself in possession of ANY M16 parts unless you have one.

Edited to add link:  www.bushmaster.com/shopping/lowers/individual_parts_for_lower_receivers.asp

Yes - FA uppers have a shallow clearance cut for "real" auto sears.

Legally speaking it is a moot point. If the lower is incapable of accepting a "real" auto sear then it is irrelevant.

The clearance is not required for DIAS or LL.

Used to be all uppers were machined to current mil spec so as to be able to sell both to gov. or open market. Now most companies target sales directly to civilian markets, and leaving that cut out saves about 10 seconds per part.

Reason I ask is eventually I`m going to get a Colt M4 upper form my Colt large hole lower.Its SA and I want to keep it all legal.

MILSPEC AR15 uppers are identical, physically. There is no way I know to tell them apart.

Some AR15 manufacturers make an AR15 specific upper. The only difference is the small machined out area at the bottom rear of the receiver. M16s need this area machined out to fit over the sear while an AR15 does not.

Bushmaster uppers have the machined out area just like M16s.

AR15 and M16s were made with and without forward assists and brass deflectors.

Someone saying you have an illegal M16 upper on your AR15 is like saying you have an M16 barrel. There is no difference between the two.

I wouldn't worry about it. - Almost all uppers have the Sear clearance cut. It is a fact that is almost never mentioned. Most people wouldn't note the difference unless you had both side by side and took the time to point it out.

edit - Just like BBB said.