It’s hard to believe the M4 Carbine has now been around for almost three decades. Colt was originally awarded a development contract for the XM4 carbine in 1985. The initial intent for the XM4 was the creation of a carbine that would serve the same purposes as the World War II M1 Carbine. That is, provide a weapon to support troops and others who did not need a full-sized M16A2. One of the stipulations in the contract was that as many parts as possible should be common with the M16A2. Another stipulation was that the new carbine would use the M855 (62-grain) round as well as the M193 (55-grain) round. The 14.5-inch barrel would, therefore, have a 1-in-7-inch twist rate. Upper and lower receivers would be the same as those of the M16A2. The XM4 would also incorporate the three-round burst feature of the M16A2.
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Due to the shorter barrel and the gas port being closer to the bolt, Colt engineers felt that the bolt would be under greater stress due to the higher cyclic rate, but to maintain parts interchangeability, there was not a redesign of the bolt or barrel extension. Some changes were, however, necessary. Because of the shape of the M855 round and the higher cyclic rate, the feed ramps on the barrel extension and the upper receiver were extended. It was also necessary to develop a new buffer to counter bolt carrier bounce during burst fire. A tungsten buffer of almost twice the weight of the M16A2’s steel one was developed to help slow down the cyclic rate. A handguard with double heatshields was also developed to allow heat to be dissipated more effectively. A few other parts were modified as well.
The M4 Carbine was initially delivered with the standard M16A2 rear sight, but was soon replaced with the Mil-Std-1913 flattop receiver, which allowed for the mounting of an array of optical sights. Both versions were supplied with the now-familiar telescoping stock.
M4s in Service
On August 15, 1994, after years of development by Colt, the U.S. armed forces adopted the M4 and M4A1 Carbines. The M4 had the three-round-burst setting, while the M4A1 had the full-auto setting. All M4A1s also had Picatinny rails. Most of the M4A1s were supplied to U.S. Special Operations Command (SOCOM). Despite the original contract specification of interchangeability, as adopted, about 25 percent of M4 components are not interchangeable with the M16A2. As a result, the Department of Defense has been very reluctant to adopt improvements requiring new parts that Colt has suggested during production of the M4/M4A1.
Because of its more compact size, the M4A1 quickly became the standard weapon for many SOCOM operators. Because of the large number of rounds shot by some SOCOM units in training (24,000 rounds per year)—especially the U.S. Navy SEALs, who were known to wear out barrels extremely quickly—problems arose with the M4A1. However, Rock Island Arsenal testing determined that the amount of full-auto shooting necessary to cause most of the problems would not occur with normal use. Colt did provide a heavier barrel designed by Rock Island Arsenal and an improved buffer for the M4A1 on later models.
As of July 2009, the U.S. Army acquired ownership of the M4 design from Colt, which will allow for the purchase of the M4 and M4A1 parts from other vendors. The Army has continued to purchase M4A1s from Colt and Remington, with contracts for well over 100,000 carbines. In February 2013, a contract was awarded to FN to produce another 120,000 M4A1 carbines. There have been some snags with the M4/M4A1 in Afghanistan and Iraq, especially with sand and dust. Problems seem to have been most common on M4/M4A1s with accessories mounted. As a result, there have been SOCOM tests of other designs, including those designed with a gas piston system rather than the direct impingement system used on the M4. However, the recent orders for M4A1s are indicative that the M4 will be around for quite awhile yet.
Daniel Defense M4A1
SOCOM does have the leeway to upgrade its M4A1s, and at least one version in use is the Daniel Defense M4A1. To backtrack a bit, though, Daniel Defense is probably best known for supplying its M4A1 Rail Interface System (RIS) II to SOCOM for the SOPMOD (Special Operations Peculiar Modifications) II program. As supplied for the SOPMOD II kit, Daniel Defense Upper Receiver Groups and Rail Interface Systems were supplied for 10.3-, 12.25-, and 14.5-inch barrels. The RIS II allows the M203 grenade launcher to be free-floated without any additional parts or special tools to mount it. This is important, as the accuracy of the M4 will not be adversely affected with the M203 mounted, though the point of aim might have to be adjusted when the launcher is mounted. The RIS II is designed to be used in conjunction with the MK12 Low Profile Gas Block, which is also produced by Daniel Defense.