One that I did recently on an 80 percent lower, (while waiting for my FA in the mail)

Tacked.

Nice!!

These were test and evaluation guns, so they would be whatever Colt put together with readily available parts...awaiting military decision of exactly what they wanted...

Sure thing.
Use my email.

That is beauty.

The M4 program was initiated (IIRC) in 1983 by 9th ID (Army) as a Carbine version of the M16A2.

USMC took it over as an SMG replacement, before it reverted back to the Army as a Carbine. "Fortunately" it retained the "M4" designation throughout because the previous weapons in each class just happened to be both the M3 Carbine and M3A1 SMG.

Near as I can tell, the "rumor" that the M4's barrel length was chosen for the ability to mount a bayonet comes from the Marine Corps' program--the 14.5" barrel length had existed since the R651 M16A1 "Carbine," and the more common R653, however when looking at SMG replacements, the Marine Corps looked at both the 11.5" "Commando" guns and the 14.5" variants, and ultimately opted to go with the 14.5" gun in part because it could accept a bayonet.

The Colt 723 and 727 barrel profile is actually slightly different than the final M4 profile--the marking was a "(C)" instead of the "C," and the "ramp" at the rear of the M203 step where the barrel goes back up to .750 is shorter and and steeper on the (C) marked barrels versus the M4 barrel--in fact, many commercial "M4" profile barrels are more similar to the (C) barrel profile than the M4 profile.

Again, as near as I can tell:

The XM4 program was the R0720 and would have been marked PROPERTY U.S. GOVT/XM4. These would ONLY have been issued to and used by DoD units for operational testing, they would not have ever been sold commercial/export or even COTS by organizations within DoD, who had been doing so for years.

Colt then began building commercial/export versions of the R0720, neverminding the BURST/AUTO variants and specific contract variants, the most commonly recognized ones are the R0723 with Field Sights -- AKA "A1 sights," while the R0727 had Match Sights -- AKA "A2 sights." These commercial/export guns were all marked "M16A2" in the commercial marking conventions, not the PROPERTY-style markings. These are the weapons that would have purchased COTS by both DoD and LE, etc., as well as FMS export weapons. Again, there were some different models with different contract variations, but they would have all come from this "family."

Once the M4 was accepted and left the operational testing phase at some point it was determined that the M4 Carbine should have a detachable carry handle with accessory rail--once the M4 became a flattop, the model number became R0920--at the time "7xx" designated fixed carry handle, while the "9xx" designated a flattop receiver.

Initially, the M4 Carbines would have gone to gaining units that by and large did not have a similar weapon that was being replaced--it was in many ways a wholly new weapon, and issued fairly sparingly to units that had a requirement for the Carbine. Obviously as GWOT came around years later, the M4 became the standard assault rifle, and now has been "replaced" yet again by the M4A1, but that brings us way out of the "A2 period."

The S-1-F SOF variant of the M4 Carbine, the M4A1, which does not appear to have gone through an operational testing period and appears to have always been in a flattop configuration (there does not appear to be any indication I've yet found of "XM4E1" or "XM4A1" marked guns being produced, though you never know with Colt...) was designated the R0921.

The M4A1 Carbines would have gone primarily to units within USSOCOM and other SOF proponents that already had COTS R072x-series weapons (usually mixed in with M16A2s), and M4A1s and R072x and even some R0777 (see below) would/could have all served along side each other during the mid-to-late 1990s when this transition was occurring.

Simultaneously, when the XM4 became the M4, Colt appears to have shifted the R0723 and R0727 family under the umbrella of R0777, dropping the R072x family. These weapons were built with a mix of R072x family parts and M4 parts, and rollmarked "M4/M16A2E" in the commercial marking conventions.

The earliest of these guns would have been almost indistinguishable from R0723s and R0727s, with "(C)" barrels, side-sling mounts, two position receiver extensions, lock nuts instead of castle nuts, single heat shield handguards, etc. Later guns would be almost indistinguishable from M4 Carbines--except have a fixed carry handle, using "C" marked M4 barrels, BUT with non-F marked FSBs, castle nuts, four position receiver extensions, etc., basically, Colt used up all their "old" parts, and then started using "new" parts--the R0777 was not discontinued by Colt well into the mid-2000s.

Also when the M4 was adopted, Colt introduced the R0977, which was the commercial/export version of the M4A1 Carbine (flattop, S-1-F). As far as I can tell, Colt chose to keep the R0977 "pure," and did NOT use leftover 72x series parts, as the R0977 became their flagship product, and was supposed to "exactly" match the military issued M4 (again--more similar to the M4A1, but they continued to call it the "M4" only).

The Israeli guns are actually primarily full-length M16A2s that were refurbished by the Israelis, who bought parts kits from Colt--they essentially re-barreled and re-stocked 20" guns. The Israelis of course are notorious for refurbishing, recycling, and even modifying M16A1s and M16A2s to suit their needs, which is why there have been plenty of "M16A1s" out there with M4 barrels as well, which would not be XM4s or R0723s or R0727s or GUU-5/Ps, as they've often been variously identified as, though the process by which they were "manufactured" is very similar to the GUU-5/P-family (for all intents and purposes, as Air Force 1/7 twist carbine... almost no other features are "standard"). The Israelis later also began purchasing complete flattop M4 uppers along with purchasing complete carbines that they would slap on whatever lower was available without concern for whether it was a commercial M16A1 or M16A2, or a "leased" PROPERTY marked gun.

Almost all Colt FMS guns (like those used by the Mexican Marines) would not be M4s or XM4s, but some combination of R0727 or 725s (S-1-3 variant), or R0777s. Very few other countries would (at the time) have received PROPERTY marked R0720s or 920s (or 921s), though again--further on into GWOT, this has changed, and there are quite a few R0920s and R0921s in foreign military use.

The side sling mount was originally not even for the XM4/M4 program, but actually a sling adapter used with the M203, which got standardized on the M4 and M4A1 and was an option on R0777 and R0977s.

There are actually several variations of the side sling swivel as well, including a simple "band" swivel, as well as the early "reversible" sling swivel (again, originally designed for the M16/M203 combination) which had a reversible bracket to match either the profile of an M16A1 or M16A2 barrel.

There was also an M16A2/M4 barrel diameter specific sling swivel with a "narrow" bracket that was constant length, and then the modern "fat" bracket, which is wider in the center (where it clamps around the barrel), and narrow at the ends where the roll pins are inserted.

~Augee