Notes: The FX-05 Xiuhcoatl (Fusil Xiuhcoatl 2005; Xiuhcoatl means Fire Serpent) began replacing the G-3 and the limited-issue M-16A1s and other assault rifles in limited use by the Mexican military in 2005. The FX-05 is itself still in limited issue, primarily due to funding problems, but it it already equipping the GAFE (Special Forces Airmobile Groups), most of the Military Police, and a few other Army units. The FX-05 has had an interesting political and legal history – most military experts believe it is an unlicensed derivative of the export version of Heckler & Koch G-36 assault rifle, with a very few modifications internally and dressed up externally so it does look exactly the same as the G-36. The Mexican firm of Mendoza (who claims to have independently developed the FX-05) was very nearly sued in international court by Heckler & Koch over this; it is possible that Heckler & Koch dropped the lawsuit in exchange for some "royalty" money and an agreement that Mexico buy an unspecified number of actual G-36 rifles from Heckler & Koch at inflated prices for their special operations units. The Mexicans did test the G-36 and rejected it, though what technical information they obtained during their trials is also unknown.

Another story says that Heckler & Koch did in fact examine the FX-05 in detail, and, while they thought it might have been based on the G-36, concluded the FX-05 was not similar enough to the G-36 for Heckler & Koch to believe they could win a lawsuit. A third story says that Heckler & Koch dropped the suit at the request of the US government, again in exchange for unspecified "rewards," and for unspecified reasons. Regardless, production of the FX-05 is proceeding at a low rate, though the Mexicans hope to go to full production rates in late 2007 or early 2008.

The FX-05 is a modular design, which allows for changes in sights, accessories, stocks, muzzle devices, etc., quite easily. Most of the receiver, optical sight/carrying handle, and the pistol grip and trigger group do in fact look almost exactly like that of the G-36, though more composites are used in the construction of those elements. (In fact, the lower receiver is virtually entirely made of high-strength polymer.) The stock is usually side-folding, though sliding stock and fixed-stock versions have been seen in photographs; the fixed and folding stocks look very much like those of FN’s SCAR, while the sliding stock is very much like that of Colt’s M-4. In the case of the side-folding and fixed stocks, they are a bit shorter than the stocks of most assault rifles, in order to account for the shorter stature of most Mexican soldiers. The handguard is equipped with short MIL-STD-1913 rails on either side near the end of the handguard; the optical sight/carrying handle can be removed, revealing another, longer MIL-STD-1913 rail atop the receiver. A possibility for future versions is another MIL-STD-1913 rail under the handguard, running the full length of the handguard (though this has not yet been seen on any FX-05). Fire controls and the magazine release are ambidextrous. The FX-05 is able to accept an M-203PI (but not a standard M-203). Standard magazines are made from translucent plastic, again virtually identical to those of the G-36, though the FX-05 is rumored to be able to accept M-16-type magazines as well (again, this has not yet been seen by observers). Rumors also state that a carbine with an even shorter barrel than the assault rifle version exist, as well as longer-barreled SAW and marksman versions; these also have not been seen by any observers as of yet.

The Mexicans have not allowed anyone else from any other country, military or civilian, to examine or even handle any FX-05s; therefore, anything after this point is essentially an educated guess. The operation is presumed to be virtually identical to that of the G-36. The barrel of the standard version has been calculated from photographs to be approximately 12.5 inches long and tipped with a pronged flash suppressor; versions have also been seen with several different types of muzzle brakes. The barrel length seems to make the FX-05 more of an assault carbine than an assault rifle. Rumors indicate that the barrel is made from stainless steel, with a chromed bore and an intermediate rifling twist compatible with both SS-109 and older M-193 5.56mm NATO ammunition (which would put it somewhere from 1:7 to 1:9). The short barreled version is rumored to have a barrel of only about 9 inches, and the barrels of the SAW and marksman versions 20 inches. (Presumably, the marksman version uses a better barrel and better sight than that of the SAW version, and the SAW version uses 100-round C-Mags as standard. Both are probably equipped with bipods. Marksman versions would probably not use muzzle brakes, but I have included them below "just in case.") Small arms experts believe the weight of the standard FX-05 to be approximately 2.65 kilograms. Assuming the FX-05 is based on the export model of the G-36, the optical sight on the carrying handle would be of 1.5x magnification with a high-contrast aiming reticle, though rumors state that the optical sight of the FX-05 does in fact have 3x magnification. Adjustable backup iron sights are also provided.

Twilight 2000 Notes: The FX-05 does not exist in the Twilight 2000 timeline.

Merc 2000 Notes: The Germans sold the Mexicans genuine G-36s instead, and the FX-05 was never even designed or contemplated.