The U.S. Army is now distributing the smaller and lighter M-4 Carbine to support troops and commanders in units not operating in the combat zone. But the M-4 isn't always as small and light as the generals think. There are now over 400,000 M-4s in use. The M16 is still the standard infantry weapon, but the shorter M-4 is replacing it in situations where weight and size is a factor, and long range shooting isn't. In some cases troops who were previously armed with a pistol, now have a more powerful M-4 to tote around.
One problem with the M-4 is that, while it is compact, it often becomes very heavy. That's because it's become customary to equip some seven pound M-4s with a three pound M203 40mm grenade launcher (under the barrel), and this is often accompanied by a quadrant sight (mounted on the left side of the carrying handle, making the M203 accurate out to 400 meters). Add one of those nifty new thermal sights for the rifle, and you have another three pounds of sights. This nearly doubles the weight of a fully tricked out M-4 to 13 pounds. The weapon is also somewhat more ungainly with all these attachments.
The M-4 has been around since the early 1990s, when a special version of the M-4 was adopted for use by SOCOM (Special Operations Command). SOCOM often takes the lead in developing new weapons, or versions of existing ones (like the M-4, a modified M-16 design). Once SOCOM has demonstrated that a new item works in combat, the army and marines are inclined to consider adopting it as well.
The M-4 is much more compact than the M-16, not just because of the shorter barrel, but because of the telescoping stock. This makes the M-4 much easier to use by people in vehicles, or for combat support people who must carry around, but rarely use, a rifle. The M-4 is 33 inches long and weighs 6.9 pounds (with a 30 round magazine.) In contrast, the M-16 weighs 8.5 pounds and is 39.5 inches long. The M-4 has a 14.5 inch barrel, while the M16's is 20 inches.
The other main difference between the two weapons is that the M-16 is more effective at longer ranges (over 300 meters), because of its longer barrel. But combat experience in the 20th century demonstrated time and again that most (over 90 percent) of the time, your average infantry soldier did not need a personal weapon that was optimized for long range shooting. Almost all combat took place at shorter ranges. It was more effective to have specialized weapons (light machine-guns and larger caliber sniper rifles) for the long range stuff, and a lighter and handier weapon for close in work.