"A3" is a name that Colt, Bushmaster, and several others use to describe flattop uppers, because "A3" was the next number after A2.  Armalite and Rock River call the same uppers "A4", since the first M16 model to have a flattop upper was the M16A4.  Don't get too caught up in the names; it's just marketing.

The only difference between standard uppers (A2 or flattop) and "M4" stamped uppers is that the latter have feed ramp extensions cut into them and are designed to match up with barrels that have the "M4" barrel extension, which has the matching extended feedramp cuts.  You should not use an M4 upper without an M4 barrel extension (or if you must, have your feedramps machined to match the upper).

-Troy

Actually you may not need to bother with matching the feed ramps based on my sample of one.

I built a M4 four years ago merely by mating a Bushmaster M4 barrel to a Colt's M4 upper.  No feed problems whatsoever with ball ammunition firing SEMI.

Try it first before spending money on machining anything.  

-- Chuck

The thing is, Chuck, you have, oh, a couple of years experience () at spotting bad magazines.  Ammo generally only hits low on the barrel extension if the mag is slightly damaged or out of alignment.  But if you were to have such a mag, you have a setup with the potential to trap the point of the bullet under the barrel extension, which results in a nice little stopage.  I've seen it happen, and I've seen uppers with wear on the upper receiver just below the barrel extension from bullet tips hitting there.  Thus, based on a broader-than-one sample, I made the above recommendation.  

-Troy