Some are saying it's the same, others say it differs.

Anyone know for sure?

I think "standard" might have a new meaning these days, as nearly every rifle I saw during news coverage of the Army's 3rd Armored Division in Iraq showed them equiped with M5 RAS's with Forward Pistol grips mounted way back on the lower rail.
Also, the 37,000+ M16A4's the Marines just bought are all M5 RAS equipped from the factory (FNMI in Columbia SC).
However, my "official" poster of "Army rifle types" dated 23 Oct. 02 does not show a RAS on the M16A4 (Army P/N: 12973001), just a flat-top upper with removeable carrying handle, and a 3-rd. burst trigger mech.
ColdBlue sends...

What about the M4.  Is a M4 (Colt) barrel assy (inc. the front sight) the same as an M4 barrel assy from, say....Bushmaster?

I remember talks about the different height of the front sight on the M4 and if that was because of the flat top upper/detachable carry handle.

I also wondered about the M16A4.

Kenny

The M16A4 uses a different barrel than the M16A2 and M16A3.

NSNs
M16A4 barrel
1005014541629
M16A2/A3
1005011467684

The difference is the FSB.

Is the different front sight base for the detachable carry handle?

Yep.

Tweak, good NSN's, I like that.  Now, what is the difference in the front sight bases?  Are both now using the M4 heigth?

Affirmative the barrels for the A4 and M4 use the "F" marked FSB.

ETA,
Before someone can ask, the M4 and M16A4 do use different flattop uppers. The M4 upper differs in having the extended feed ramps common to the M4.

ETA2,

The M16A2/3/4 all use the same front sight post while the M4 uses a different one. The M4 post is thinner to maintain the correct relationship with the shorter sight radius of the M4 barrel.

The M16A2 and M16A3 share the same barrel, the M16A4 is different (as noted above).

The NSN for the new, heavy barrel for the M4A1 is 1005-01-471-5456.  Change 7 to the -23&P shows these parts, I've not seen an on-line version of the TM which includes Change 7 as yet.

-- Chuck

Would that be the M4 profile with an extra 4 oz of barrel material under the handguards?

That would be the M4A1 barrel heavy under the handgaurds, probably 4oz to 1/2 a lb heavier.

Friend of mine picked up a Colt's M16A4 upper that has a M4 upper receiver, and the barrel has M4 feed ramps (lucky puke, he always finds stuff like that).  The upper is a later one with the numbers on top, and the flash hider has the newer crush washer.  I would GUESS it dates from the late 90's in that it has a C A marked upper, rather then a Diamaco upper.  Very slick set up, can't get him to sell it though.  It also has a Colt's carrier but a FNMI bolt.  I have also seen commercial MT6700s with M4 uppers and M4 feed ramps in the barrel.  Looked two over at the last show, both were in the 36,000 serial number range, funny thing was the older one (CA marked, and no numbers) had a M4 upper and the newer one did not.

Am well aware that the Colt's M16A4 came (past tense?) without a M4 upper, just so happens a friend of mine sent me some pics of a Govt issue mid 90's Colt's M16A4.  Guess I need to post them up some time, great detail shots and it has a standard upper.  Also sent me a picture of an FNMI M16A2 lower, because I never saw one while I was in.  Kinda of disappointing, real plain Jane markings.

Any how, I am going to update my M16A4gery with an M4 upper, don't know where I am going to get the barrel, guess I will have to cut the feed ramps in the extension myself?  Or maybe wait till they are easier to find?

I did some measuring of non F marked and F marked front sight bases myself (being the curious type), and could not find a measurable height difference.  There was more variation in height from sight base to sight base, then from the F's and the non F's.  Maybe I had to small of a test group?  

I just love modern small arms variation trivia.  USGI M16 variation is boring compared to those Russians who change about every dang part on their AK-74s every couple of years, darn head scratcher that is.  Am I a rambling again?  

Sorry Chuck, that has been an ongoing project, but lower in priority to my other projects.  It will get done (eventually).