The rejected U.S. Assault Rifle, the XM8, is a modular weapon that can be fitted with three different barrels (9", 12.5", 20".)
The 9" barrel is for a very compact weapon for tank crews or commandoes. With the nine inch barrel and the butt stock retracted, the weapon has an overall length of 21 inches.
The 12.5" barrel is the standard, for use with the assault rifle or, with the butt stock retracted, it serves as a carbine. The 20 inch barrel is thicker as well and used for the light machine-gun version. There is also a lighter 20 inch barrel for use by snipers.
The assault rifle can be equipped with the new M320 40mm grenade launcher.
The U.S. Army is happy with the initial field testing of it's new M-8 (or XM-8) Assault Rifle. One of the major design features of the M-8 that makes it superior to the M-16 is the way it handles propellant gasses. The M-16 has these gasses going into the receiver, depositing layers of crud from propellant that did not completely burn. The M-8 keeps the propellant gasses out of the receiver and this reduces the cleaning time by about 70 percent. The troops appreciate this. More importantly, the reduced amount of crud in the receiver greatly increases reliability (far fewer rounds getting stuck.) In fact, the M-8 is designed to fire 15,000 rounds without cleaning or lubrication, even in a dirty (like a desert) environment. Troops are not allowed to let their weapons go like that, but this degree of reliability makes it less likely that rifles won't jam in a sandstorm or after getting dropped in the mud. The M-8 barrel and receiver is also of more sturdy construction, making it less likely that the user will get injured if there's something in the barrel when a round is fired. This is not unusual in combat. All you have to do is accidentally jam the barrel into the dirt while hitting the ground or otherwise avoiding enemy fire, and then have to return fire. On an M-16, this can often cause the rifle to, well, blow up in your face. This unfortunate event is much less likely with the M-8.
The M-8 comes with a battery powered sight that includes a red-dot, close-combat capability, plus infrared laser aimer and laser illuminator with a backup etched reticule. The sights on the M-8, similar to those which have been showing up on M-16s over the past decade, make it much easier to hit something. The M-8 is better designed for "ease of use" and support troops who don't handle their weapons frequently will find that they can more easily hit something with an M-8. Tests, using people who have not handled a rifle frequently, have demonstrated this.
Because the attachment points for rail mounted devices are built into the M-8, the sight can be factory zeroed. The M-16, because it has rail mounting hardware mounted on it, requires frequent re-zeroing in the field. This is a feature very much appreciated by the troops. The attachment points allow additional sighting devices to be quickly added to the weapon. A new 40mm, single shot grenade launcher (the M320) will be available for the M-8 and can be quickly installed by troops, without special tools. The M-8 is designed for easy left or right handed operation.