https://en.wikipedia.org/wiki/5.56%C3%9745mm_NATOтук има добро описание - кога какво са приемали на въоръжение, кое и какво са прoменяли през годините в Американската армия и най-вече аргументи защо
55grn US Army
Cartridge, Caliber 5.56 mm, Ball, M193: 5.56×45mm 55-grain [3.56 g] ball cartridge. This was type-standardized and designated by the US Army in September,
1963.62grn SS109/M855 ...
In 1970M855A1 Enhanced Performance Round, причинява някой проблеми на буталните М27 (пуснахме в темата цяла статия за това)
ХК прави някой промени, въвежда подобрен газов регулатор, и т.н. и тогава приемат M855A1 .. за буталките, иначе ги троши яко, особено 14ките, 10ките понясят по добре М855А1
М855А1 е оптимизиран за по къси цеви, по бърз барут, по малко пламък, по големи налягания и т.н.
даже и за газовите със CARBINE газова система идват силнички особено + супресор .. и SOCOM новият им ъпдейт е 14ка с мид газ, англичаните са 13,8инча с мид газ и супресор
77грнMk 262
The Mk 262 is a match-quality round manufactured by Black Hills Ammunition made originally for the Special Purpose Rifle (SPR). It uses a 77-grain (5.0 g) Sierra MatchKing bullet that is more effective at longer ranges than the standard issue M855 round.
In 1999, SOCOM requested Black Hills Ammunition to develop ammunition for the Mk 12 SPR that SOCOM was designing. For the rifle to be accurate out to 700 yards, Black Hills "militarized" a cartridge that used the Sierra 77 grain OTM (Open Tip Match) projectile; it switched from a .223 Remington to 5.56 mm case, increased pressure loading, crimped and sealed the primer, and added a flash retardant to the powder. The Mk 262 MOD 0 was adopted in 2002. Issues came up in development including reliability problems in different temperatures and when the weapon got dirty, and cycling issues in cold weather due to the slightly shorter barrel of the SPR compared to the full-length M16A2 barrel. The problems were addressed with a slower burning powder with a different pressure for use in the barrel, creating the Mk 262 MOD 1 in 2003. During the product improvement stage, the new propellant was found to be more sensitive to heat in weapon chambers during rapid firings, resulting in increased pressures and failure to extract. This was addressed with another powder blend with higher heat tolerance and improved brass. Also during the stage, Black Hills wanted the bullet to be given a cannelure, which had been previously rejected for fear it would affect accuracy. It was eventually added for effective crimping to ensure that the projectile would not move back into the case and cause a malfunction during auto-load feeding. Although the temperature sensitive powder and new bullet changed specifications, the designation remained as the MOD 1.[154]
According to US DoD sources, the Mk 262 round is capable of making kills at 700 meters. Ballistics tests found that the round caused "consistent initial yaw in soft tissue" between 3 and 4 in at ranges from 15 feet to 300 meters. Apparently it is superior to the standard M855 round when fired from an M4 or M16 rifle, increasing accuracy from 3–5 minutes of angle to 2 minute of angle. It possesses superior stopping power, and can allow for engagements to be extended to up to 700 meters when fired from an 18-inch barrel. It appears that this round can drastically improve the performance of any AR-15 weapon chambered to .223/5.56 mm. Superior accuracy, wounding capacity, stopping power and range have made this the preferred round of many special forces operators, and highly desirable as a replacement for the older, Belgian-designed 5.56×45mm SS109/M855 NATO round. In one engagement, a two-man special forces team reported 75 kills with 77 rounds.[155] The Mk 262 has a higher ballistic coefficient than the M855 of (G1)0.362 / (G7)0.181, meaning it loses less velocity at long-range.[143]
Black Hills also produces the MK262 Mod-1C for sale in the US civilian market under Product Code D556N9.[156]
и съвсем
нов 62grnспоред доклади .. M855 не е ефективен за трепане и пробиване на жилетки
и сега тестват нов .. 62 грн .. Mk318 MOD 0 "Cartridge, Caliber 5.56mm Ball, Carbine, Barrier"
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. 5.56 NATO e много добре балансиран патрон откъм тегло спрямо спрямо енергия, пробивност и далекобойност. Бъдещето на този патрон е с тежки 77 грейна проектили с висок балистичен коефициент, които са изключително точни дори на сериозни дистанции над 400 метра. 
... и старите кадри още воюват някъде във Виетнамската ера