So can someone line out the real differences between these to carbines?  What are the nuances of each?  It doesn't look as if there are any huge difference, so maybe just a lot of little details?
What's the scoop?
Ralph

Ok, I'll try and take a stab at this...

First off, let's be sure we're on the same page - I'm going to assume that by "727" you are referring to the Colt model R0727, and by "XM4" you're referring to the Colt model R0720, rather than the Colt model R0920 which would be a straight up M4 Carbine (with or without the RAS).

As per the Colt Model Guide of The Black Rifle II (strangely found on page 272), the Colt model R0727 is listed as having an M4 profile 14.5" barrel; "Auto" fire control; an "A1M" (C7-style) upper receiver, using an A1 rear sight; an A2 front sight; and is further listed as using "M4 and Carbine" hand guards, though honestly, I'm not sure what is meant by that last part.

Whereas...

The Colt model R0720 (XM4) is listed as having a 14.5" "A2" profile barrel; "Burst" fire control; an "A2 with M203" upper receiver; A2 front and rear sights; and M4 hand guards.

Stocks for both models are listed as "Sliding", though it is known that the early XM4 utilized an N1-style fiber-lite stock on a 4-position receiver extension.

You should really get yourself a copy of The Black Rifle & The Black Rifle II, and read the sections on the development of the M4/M4A1 as there were so many changes going on at that time, it's rather difficult to nail-down an exact borderline between many of the various models.

Hope this helps...

And since we opened that can of worms.....Let's not forget the input from TBR on this subject.    So, was the so called "Abu-Dabi" Carbine related to the XM4??   The author seemed to think so.  Others opine that real XM4s must be flatties, or they're fakes, FARBS, and patent infringements !   As for Colt's model designators, this is  a study in confusion all it's own !    

So if I'm following this, there is some overlap and disagreement with what is or is not, a 727/XM4.  I have seen 727s that have A2 uppers and like wise reputed XM4s that have the same fixed handle receivers.
Thanks,
Ralph

make the one that calls to you the most :)

I was going to point you to the long 727 thread, but it looks like you started it too.

Augee to the white courtesy phone...

When I built my 727 a couple years ago, I went with an A2 upper receiver, A1 lower receiver, 6-hole CAR handguards, A2 pistol grip. A2 FSB, Colt 14.5" barrel w/ 203 reliefs, Ext. A2 FH, 2-pos. receiver extension, and fiberlite stock. I am very happy with this build.





Here is a link to what I used for a reference for my build.
727

I did exactly the same thing, but was able to find a gray Colt A2 upper marked XM4 above the gastube hole

Uh oh.  I don't think I ever heard of that before.  There was some debate whether or not M4's were truly all flat tops or if some were actually not flat tops.  Or maybe just because it was stamped "XM4" that might make a carry handle OK.  I don't know.

Or someone stamped it themselves.

Can't say for sure, It probably passed through a dozen or more hands in 20-25 years before I found it. I like it though....

Well right you are!
Seems I may have forgotten that.  Thanks for the reminder.  It got so big so fast I lost track of it while I was ill.
Checked it out again, all the answers and yet more questions.....
Ralph

This is what what I had in mind.  Thanks so much for the link.
Ralph

When I did my XM4, I went with the 4 position extension, current castle nut, A2 lower and according to most recent info probably should have the fat hand guards.

The XM4 project was initiated by the USMC in the early 1980s, then quickly picked up by Army Ordnance, originally intended as a replacement for the M3 SMG.  

The XM4, as far as I have been able to determine carries the model number designation "RO720."  

The RO720 went through several different forms during its development, including at least four different barrel profiles, including a .625 "pencil" barrel, an HBAR profile, a profile similar to the final M4 and the Sig M400, but with a single "step" in the barrel (thicker up front), and narrow all the way to the FSB, and the final M4 profile barrel.  

The final configuration of the XM4/RO720 should be nearly identical to the early RO920 M4 Carbine with the exception of being roll-marked "XM4 CARBINE" and having a fixed carry handle upper receiver using target (A2) sights.  

In every other way, it should be identical to the appropriate vintage of M4 Carbine -

Lt. Col. Lutz (coldblue) recently posted photographs of one of the last XM4 prototypes before M4 production began - the only difference is that it did not have a side sling swivel, and retained the "standard" 6 o'clock sling swivel.  

Otherwise, the late XM4/RO720 should have:

True M4 profile barrel and A2 compensator timed with a peel washer

True M4 feedramps in the barrel extension

Castle nut and four position receiver extension, and fiberlite CAR stock

Double heat shield "M4" handguards

"U.S. GOVT PROPERTY" rollmarkings.  


The RO727 was a parallel development to the XM4, though in its final configuration, it looked superficially very similar.  

The RO727 was a version of the M16A2, and thus appropriately rollmarked as such, with no "U.S. GOVT PROPERTY" markings, and the stylized, commercial "COLT" logo.  

Some evidence suggests that at least some RO727s came with lightweight 1/7 twist barrels with .625 gas block/FSBs, similar to the RO723.  There are also a couple of photographs that appear to show RO727s with vinyl-acetate coated aluminum stocks as well, but these can not be confirmed.  However, again, early RO723s did have vinyl-acetate coated stocks, so it is hypothetically possible for an early RO727 to have had one.  

Nevertheless, the most commonly seen variant of the RO727 used a fiberlite CAR stock, and an M16A2 profiled barrel, machined to clear the mounting hardware of an M203.

While the RO727's barrel, as well as other carbines that used this barrel (RO723, RO725, RO729) superficially resembles and to a casual observer, looks identical to an M4 barrel, it is not the same barrel.  The profile of the clearance cut is slightly different between the two barrels - and 7xx series carbine barrels were marked with a "(C)," in parenthesis.  

The feedramps in the barrel extension were also slightly shallower.  

Likely, the 7xx series barrel profile was an early version of the XM4 barrel that "crossed over" to commercial/export production before being fully refined.

The 7xx series also used the same receiver extension and lock ring assembly that had been in use since the R609 (XM177), two positions, with a narrow lock ring with circular holes.  

The 7xx series also appears to have exclusively used single-heat shield "six hole" CAR handguards throughout their production.  

Moreover, Colt was know throughout their commercial/export M16A2 production to use both A1 and A2 lower forgings, and many lower receivers were in fact "pre-marked" "SAFE/SEMI/BURST," even when S-1-F FCGs were installed from the factory.  

All would have had commercial M16A2 markings, however.  


Once the M4 was introduced, Colt appears to have discontinued the production of the RO727, and switched their commercial and export production to the RO777 (and its S-1-3 sister, the RO779).  These would have been nearly identical to the RO727 and RO729, however, they would have begun to integrate M4 components in lieu of RO727 parts until they were gradually all used up.  

The RO777 series may have consolidated both the RO723 and RO727 models into a single model number, however, I have been unable to fully substantiate it - there may be another set of model numbers I have not yet found corresponding to the A2 Field Sights (A2/FS) of the RO723/5.  

The rollmarking for the RO777 and RO779 was commercial/export-style, and read "M4/M16A2E."  

Every lower I have seen bearing this rollmarking has been an A2 forging, however, it would be impossible to rule out for certain than none used A1-style forgings like the RO727.  

The RO777 (fixed carry handle) would correspond to Colt's model numbering practices from that era as well - as the RO777 and RO779 would correspond to the flat-top commercial/export variants: RO977 (S-1-F M4 Carbine) and RO979 (S-1-3 M4 Carbine).  

RO720 (fixed carry handle U.S. Government XM4 S-1-3) would also correspond to RO920 (flat-top U.S. Government M4 S-1-3).  

The debate which exists about the configuration of the first run of USGI M4 Carbines is whether or not the first carbines rollmarked on the receiver "PROPERTY OF U.S. GOVT M4 CARBINE" (not XM4) were fixed carry handle models, or flat-top receivers - and if they were fixed carry handle models - whether they were designated as RO720s or RO920s, basically becoming a "chicken or the egg" question.

As far as I can tell, the Colt model RO777/779 was discontinued around 2003, but don't quote me on the exact date there.  

~Augee

This un-muddies it a bit for me. Thanks Augee.
Ralph

Some reference photographs:

Early RO720 (XM4) with "M400-Style" barrel -




coldblue posted "late" XM4 (RO720) -




autoweapons RO727 - side sling swivel likely not original - note A1-style lower forging -




Colt commercial/export M16A2 marking:




Colt RO777/779 "M4/M16A2E" rollmark:




~Augee

I bet this soldier learned real quick not to place his thumb where he did...

Actually, it appears to have started as an Army program that the USMC latched upon, then the Army abandoned it.  After letting the USMC slog around for a few years, the Army jumped back in.

The 9th Infantry Division requested a Quick Reaction Program for a 5.56mm carbine as early as April 1983.  It was originally framed in terms of a modified XM177E2 with improved furniture and a 1-7" barrel.  The Army's Armament Research and development Center (ARDC) reviewed the QRP in June 1983 and noted that the XM177E2 would need additional modifications beyond those cited by 9ID.  ARDC recommended additional commonality with the M16A2, as well as lengthening the barrel to 14.5".  In January 1984, 9ID revised the QRP and redesignated the proposed 5.56mm carbine as the XM4 Carbine.  The Army formally approved the revised QRP in February 1984.

This rings a bell somewhere in my addled mind...  

Thanks for the clarification and correction.  

~Augee

Here is my 727 Clone