M4 Carbine

Visible and IR (infrared) lights of various manufacturers are also commonly attached using various mounting methods. As with all versions of the M16, the M4 accepts a blank-firing attachment (BFA).
Effectiveness
An April 2002 presentation by the US Army Natick Soldier Center presented by LTC Charlie Dean and SFC Sam Newland reported on lessons learned from M4 use in Afghanistan (such as use during Operation Anaconda):
90% of soldiers reported confidence in the weapon.
20% were dissatisfied with its ease of maintenance.
34% of soldiers reported that their M4's handguards rattle and become excessively hot when firing.
15% reported that they had trouble zeroing the M68 reflex sight.
35% added barber brushes and 24% added dental picks to their cleaning kits.
Soldiers reported the following malfunctions:
20% reported double-feeding.
15% reported feeding jams.
13% reported that feeding problems were usually due to magazines.

Soldiers requested the following changes:
55% requested the firearm be made lighter
20% requested a slightly larger magazine
2007 dust test
In the fall of 2007, the Army tested the M4 against three other carbines in "sandstorm conditions" at Aberdeen Proving Ground, Maryland: the Heckler & Koch XM8 rifle, Fabrique Nationale de Herstal SOF Combat Assault Rifle (SCAR) and the Heckler & Koch HK416.

Ten of each type of rifle were used to fire 6,000 rounds each, for a total of 60,000 rounds per rifle type. The M4 suffered far more stoppages than its competitors: 882 stoppages, 19 requiring an armorer to fix. The XM8 had the fewest stoppages, 116 minor stoppages and 11 major ones, followed by the FN SCAR with 226 stoppages and the HK416 with 233. The Army was quick to point out that even with 863 minor stoppages—termed "class one" stoppages which require 10 seconds or less to clear and "class two" stoppages which require more than ten seconds to clear—the M4 functioned well, with over 98 percent of the 60,000 total rounds firing without a problem.

Civilian models are sometimes colloquially referred to as "M4gery" (pronounced ĕm'fôr jə-rē, a portmanteau of "M4" and "forgery"). Other manufacturers had long maintained that Colt had been overstating their rights — "M4" has now become more of a generic term for a shortened M16/AR-15.

In April 2004, Colt filed a lawsuit against Heckler & Koch and Bushmaster Firearms, claiming acts of trademark infringement, trade dress infringement, trademark dilution, false designation of origin, false advertising, patent infringement, unfair competition, and deceptive trade practices. Heckler & Koch later settled out of court, changing one product's name from "HK M4" to "HK416".

citizen ownership
Sales of select-fire or full automatic M4s by Colt are restricted to military and civilian law enforcement. While many machine guns can be legally owned with a proper tax stamp from the Bureau of Alcohol, Tobacco, Firearms and Explosives, an amendment to the Firearm Owners Protection Act of 1986 barred the transfer to private citizens of machine guns made in the U.S.http:www.bdmilitary.com/index.php?option=com_content&view=article&id=210&Itemid=95.