November 28, 2015:
The U.S. Marine Corps has decided to equip all their combat troops with 17,000 M4 carbines and relegate their M16A4 rifles to support troops .Eventually all M16s will be retired and replaced with M4s. This came after fifteen years of debate within the Marine Corps over proposals to replace the M16 with the M4. What finally settled the issue was the extensive combat experience (mainly by soldiers) with the M4 and the few marine units that have been using the M4. There was also a recent competitive test between the M4 and M16A4 and because of improved Mk318 ammo the M4 outperformed the M16A4 at all ranges. For most marines the M4 was already a favorite because it was more compact (and easier to handle when getting out of a vehicle) and lighter (a big plus when on foot). All this led to the reversal of a 2002 decision to not adopt the M4 and keep the M16A4.
A key element in all this was the ammo and recent improvements in 5.56mm ammunition. Shortly after 2001 SOCOM and the marines, responding to complaints from troops that the standard 5.56 and 9mm full metal jacket bullets were not doing enough damage to stop fanatical Islamic terrorist fighters, began issuing hollow point bullets and troops were satisfied with the improved stopping power. As a result the Marines switched over to the MK318 hollow point (or “open tip”) round for its assault rifles and machine-guns. The existing M855 full metal jacket rounds will be used up in training exercises. SOCOM has also switched to hollow point for pistols (9mm and 11.4mm) and rifles. There is a popular and long-standing myth that hollow point bullets (which expand on hitting and create larger and more damaging wounds) are illegal according to the Geneva Convention. That treaty does not mention bullets. The later 1899 Hague Convention does and it prohibits some types of hollow point bullets. The U.S. never signed the Hague Convention and was never bound by it. But in deference to allies who did sign American forces have rarely used hollow point bullets. One frequent exception was for certain types of commando operations, like hostage rescue. No allies ever complained about this. This new ammo and the current M4A1 proved to be a better rifle for soldiers and marines.
Meanwhile the U.S. Army has gone ahead with developing another M4 upgrade. This will turn M4A1s into the M4A2 (or M4A1+, take your pick). The list of M4A2 improvements is still being compiled from user suggestions and analysis of what is affordable. Most likely improvements are a longer (by a third) Picatinny rail (to make some accessories easier to use), a floating barrel, a single stage trigger (preferred by snipers), a more effective flash suppressor, camouflage colored components (to make the currently all black rifle less easy to spot), better cleaning tools (more like commercially available stuff) and some design changes (different shapes and materials for some components plus more efficient slings) to make the M4 easier to handle and carry. The "floating" barrel means that the barrel is attached only to the main body of the rifle to reduce resonance (which throws off accuracy) when it is attached to the stock. The new trigger would only be on M4s used by marksmen (about ten percent of all infantry) who are trained to be snipers.
The marines aren’t the only ones seeking to upgrade their assault rifles. In 2013 the army held another competition to find a replacement for the aging (1980s vintage) M4 carbine. There has been a lot of pressure from the troops and Congress (where many of the complaints end up, after all, the M4 users are also voters). The basic infantry weapon, the M16, is half a century old and the M4 is a shortened version of that. The rifle the M16 replaced (the M14) lasted less than a decade and the one before that (the M-1) lasted two decades. The one before that (M1903) lasted three decades and so on, back to the Civil War (1861-65). The M-4 is a variant of the M-16 that was adopted in the 1980s, but there are still plenty of M-16s in service. The army declared the competition inconclusive and went for yet another upgrade of the M4.
The army introduced the M4A1 in 2011. The army bought upgrade kits for 70 percent of the army’s half million M4 carbines and turned them into M4A1s. This conversion is still underway and will not be complete until 2020. The kits replace the barrel, receiver, and auto-loading system with one that is easier to keep clean. There is also a heavier barrel and the ability to fire full automatic. There are also a stronger Picatinny rail on top of the barrel, for mounting scopes and such.
Most elements (except for the piston loading system) of the M4A1 were already incorporated by SOCOM (Special Operations Command) for their own M4s, which were, in effect, the first M4A1 models to enter service. This conversion kit addressed years of complaints about the M4 and M16 assault rifles. The main change was replacing the main portion of the rifle with a new component that contains a short stroke piston gas system (to reduce buildup of carbon inside the rifle) and a heavier (by 142 gr/five ounces) barrel (which reduces barrel failure from too much heat, which happens when several hundred rounds are fired within a few minutes).
In 2002 the marines decided to stick with the M16A4 rather than the lighter M4. Since 2000 the marines had been testing the lighter M4. At the time the M16A4 proved to be more reliable than the M4 in tests. Meanwhile marine special operations troops had been using the M4 since 1999 and preferred them because of the more compact size and lighter weight. The two weapons are nearly identical except for the M-4s lighter weight (6.5 pounds versus 7.5) and lower accuracy and hitting power of M-4 bullets beyond 200 meters. But over the next decade the M4 caught up with the M16 in all areas and surpassed it in some.