A firearm derived from the M16A2/A4, commonly thought to be an extremely reliable and "1337" weapon. In actuality, the M4A1 (and M16 line, for that matter) requires constant cleaning and maintenance, and is extremely susceptible to jamming from sand and dust. Careful maintenance, however, can negate these negative effects.
It should also be noted that the M16/M4 fires the 5.56x45mm NATO cartridge, a bullet designed to WOUND, not KILL a target. The 5.56mm's lack of stopping power is proving problematic when faced with less squad-oriented enemies (this was noticed particularly in the 1993 conflict in Somalia and today in the Middle East). The 5.56mm was designed to basically incapacitate three men with one shot-- the wounded soldier, the medic that's supposed to work on him, and the soldier's friend/medic's assistant. Now, however, UN and NATO forces are facing insurgents that don't actually field treat dead or wounded.
"In July 2007, the US Army announced a limited competition between the M4A1
carbine, FN SCAR, HK416, and the previously-shelved HK XM8. Ten examples of each of the four competitors were involved. Each weapon was fired for 6,000 rounds in an "extreme dust environment." The purpose of the shootoff was for assessing future needs, not to select a replacement for the M4.34
The XM8 scored the best, with only 127 stoppages in 6,000 total rounds, the MK16 SCAR Light had 226 stoppages, while the HK416 had 233 stoppages. The M4 carbine scored "significantly worse" than the rest of the field with 882 stoppages. The results have been thrown into question due to the M4 scoring 307 jams in a previous dust chamber test conducted only a few months before with the same conditions and lubrication."