A look at combat rifles

The Associated Press

One of the biggest differences between Colt's M4 carbine and its competitors is the way the bullets are fed through the rifles. The HK416, XM8 and SCAR use a gas piston system to cycle the bullets automatically. The M4 uses "gas impingement," a method that uses a tube to push hot gas through key parts of the gun. This leaves residue behind, detractors say, and makes the M4 more prone to jamming. But Army officials and Colt executives say they've received no significant complaints from troops in Iraq and Afghanistan.

M4: Colt Defense's M4 carbine is a direct descendant of the company's M16 first used during the Vietnam War. At 33.6 inches long and weighing 7 1/2 pounds when loaded, it is shorter and lighter than the M16 but shoots the same 5.56 mm round. (The round is about the size of an AAA battery.) The M4's compact design makes it ideal for troops traveling in Humvees or fighting in confined areas. An M4 costs about $1,500. The weapon is used by all the U.S. military branches.

HK416: Designed by Germany's Heckler & Koch, the HK 416 carbine is slightly heavier than the M4, but otherwise similar in appearance and feel. Heckler & Koch advertises its weapon as more rugged and accurate than the M4. At $1,425 each (2007 prices), the HK416 costs about the same as the M4. It also shoots a 5.56 round. Elite U.S. military units such as the anti-terrorist Delta Force are using the HK416. Norway selected the rifle last year for its military forces.

XM8: Also designed by Heckler & Koch, the XM8 weighs 8.3 pounds and fires 5.56 mm ammunition. Several years ago it was being pursued as a replacement for the M4s in the U.S. military's inventory. The project was halted in 2005, however, due to questions over how the program was being handled. No pricing data is available.

SCAR: The acronym stands for Special Operations Forces Combat Assault Rifle. Made by Belgium's FN Herstal, two versions of the weapon — one fires the 5.56 round and another shoots a heaver 7.62 mm cartridge — are being purchased by U.S. Special Operations Command in Tampa, Fla. The lighter SCAR weighs 8.5 pounds loaded. The command, which has its own budget for unique gear, says the SCARs are more durable and accurate than the M4. The command and FN Herstal are negotiating costs, but they expect the light model SCAR to cost about the same as an M4.