What (if any) is the difference between an A3 and an A4 upper?  Is this simply marketing drivel amongst the civilian manufacturers?

Pretty much.

I thought the A4 was a 20" Government profile, A2 front sight tower, bayo lug, full A2 handguards and flattop upper?

I beleive the A4 has the same feed ramp cuts as the M4.  This thread could use a little help from one of the local experts.

If I remember correctly, civilians termed the flat top upper the A3 after the fixed carry handle versions A1 and A2.  The military calls the flat top configuration A4 (as well as other aspects), but that was after A3 was bandied about.  In any case, A3 and A4 both refer to flat top configuration as far as civilians are concerned.  I may need correcting though.

Civilian flat-tops are either A3 or A4
A1 (which is military has no brass deflector or forward assist...only sight is windage on the rear and elevation on the front post adjustment)
C7/C8 (Canadian and US civilian style A1 with brass deflector and forward assist)
A2 is civilian and military fixed sights with rear capable of windage and elevation adjustmet)
A3 is really a military designation to show that an A2 style M16 is full auto and not tri-burst....I think I'm right on that one.)  In the civilian world we refer to it as the flat-top.
A4 is military designation for flat-top, same as civilian.

So yes in the civilian world they are the same.  Hope that helps...feel free to add or correct my post.

-e-

This simply marketing drivel amongst the civilian manufacturers.  And not consistent across the market.  

Regarding the M16 (emphasis on the upper):

XM16/M16 (produced in quantity only for USAF over 40 years ago) are the only models lacking Forward Assist.  Simple flip rear sight (incorrectly called an A1 rear sight) adjustable for windage only.  SAFE-SEMI-AUTO.  Colt's SP1 models mimic this very well.

XM16E1/M16[b]A1[/b] added Forward Assist among other enhancements.  Forward Assist is the [i]defining[/i] item for an A1.  SAFE-SEMI-AUTO.  Fixed handle.

M16[b]A2[/b] added the brass deflector bump and Camp Perry rear sights.  SAFE-SEMI-BURST.  Fixed handle.

C7 made for Canadian (and Danish) Forces is a mix of the M16 handle and sights with the brass deflector of the M16A2.  Marketed as a A1 type by many, but it's not.  Fixed handle.

M16[b]A3[/b] is a M16A2 with SAFE-SEMI-AUTO.  Fixed handle, of course.  No differences in the uppers.

M16[b]A4[/b] is a M16A2 with a detachable handle.  SAFE-SEMI-BURST.  This receiver has a different part number from the similar flattop M4 receiver, but this [i]may[/b] be because the M4 is still a proprietary Colt's product.

The only adopted models of the carbine are the M4, SAFE-SEMI-BURST; and the M4A1. ...-Auto.  All but the first few have detachable handles, there's no M4A2, M4A3 etc.

-- Chuck

OK, where does M4 come from, and what is it's definition? Is it a A3 with removable carry handle and flat top upper when the carry handle is removed?

An M4 is the M4 [b]carbine[/b] just like the M16 is the M16 rifle.  It has to do with the whole weapon (14.5" M4 profile barrel, collapsing stock etc) some are flat top and some are not.  Some are 3 shot bust some full auto. You have M4, M4A1, etc. its a whole other thing.


Then there is the uppers marked "M4" or "4" by Colt which is a marking they use on some of their flat top uppers. Confusing isnt it?

[url=groups.msn.com/TheMarylandAR15ShootersSite/whatisana1a2a3ora4.msnw]What is an A1, A2, A3, or A4?[/url]