Saw a M4 upper on ebay .The ad says it is a A4 upper with keyhole forging mark,not a A3 so no feed ramps.What is the difference of A3 to A4 upper? And what is a keyhole mark for.Thanks  

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2193294716&category=36074

The keyhole mark is the manufacturing ID of the forge that turned out the raw blank. Doesn't mean a damn thing, since all the aftermarket makers are currently using blanks turned out by this forge. Keep meaning to write down the forge name when it gets posted, but keep forgetting to [;D]

My impression is that A3,A4 are differences in terminology between the military and civilian market designations, but if I'm incorrect, I'm sure someone else will chime in.

My advise, for what it's worth, is; if you want to buy an upper, look at the EE section of this website. You'll see nothing on ebay that you can't find here, and you have an infinitely better shot at dealing with an honest person on this site.


(edited to add) Just spotted the forge name on another post.  It's Cerro Forging.

The M16A3 model is like the M16A2.  The only difference is that the M16A3 is equipped with semi/auto settings on the selector, while the M16A2 has semi/3 round burst settings.  The military M16A4 is like the M16A2, but is equipped with the flat topped upper receiver incorporating the Picatinny Rail. Commercial rifle makers adopted the A3 designation for their flattops without knowledge of the military plans.


 

ALSO - Armalite uses the A4 to designate it's
rifles and uppers that have a detachable
FRONT sight.

Yes, they do. They (Armalite) are one of the vendors that use the the "A4" designation in a way that makes sense (for their flat top rifles).