Never underestimate a round, only because it's .22 diameter bullet.
Graphic Pictures :
This is work of NATO 5.56mm which are lighter than OFB 5.56mm ammo
http://lh6.ggpht.com/_vJcKa90vLkg/S2RTg_KWWCI/AAAAAAAAHyI/JkspbO3VNfU/s1024/image006.jpg
http://www.gunslot.com/files/gunslot/images/75688.jpg
sniper_shot2_292.jpg Photo by IkeMcgowan | Photobucket
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5.56mm has many variation, IA use 64gr 5.56mm with Muzzle velocity of 2900fps vs NATO SS109 62gn at 2900fps, With 1B1`s 18inch barrel and with twist rate 1/7, Its is killing even at 650ms range, 600ms is more than sufficient for AR range as most battle occur under 300ms ..
INSAS 5.56mm Rifle Use NATO standard FMJ Ammunition this Include SS109, M193 Also Indian Made 5.56mm, Details and Pictures following >>
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5.56 X 45MM SS109 FMJ
Mass : 4.0 gm
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5.56 X 45mm M193 FMJ
Mass : 3.56 gm
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5.56 X 45mm OFB FMJ
Mass :
4.16 gm
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About newer 6.5mm or 6.8mm and their marketing advertise as it is heavier and faster yet at affordable recoil, Then its just advertisement for marketing, New Generation 5.56mm have similar or better preform ace, I have posted a research in last page conducted by US army with other caliber rounds as you mentioned ..
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The U.S. Army recently conducted one of the most extensive studies into small arms performance in CQB (Close Quarters Battle) and published their findings. This study was conducted to answer concerns that some warriors were expressing in After Action Reports (AAR) post battle about the effectiveness of the 5.56mm NATO cartridge.
The test team not only tested M193, M995 (AP) and M855 "Green Tip", but they also tested a number of other loads to see if there were any improvements made to the combat effectiveness of the 5.56mm round by civilian companies.
In the 5th paragraph of the report, Major David LaFontaine makes the following statement which pretty much sums up what I've been saying about caliber selection for many years.
'In the end, "footpounds of energy" is misleading, "stopping power" is a myth, and the "oneshot drop" is a rare possibility dependent more on the statistics of hit placement than weapon and ammunition selection. Effectiveness ultimately equates to the potential of the weapons system to eliminate its target as a militarily relevant threat."
In the end the U.S. Army found that no commercially available alternatives in 5.56mm ammunition performed measurably better than existing issued ammo (M855, M193, M995).
This study was based on CQB effectiveness, and from the ranges of 0-50 meters all ammo tested performed similarly and none stood out as being clearly superior.
Also worth mention is that during this testing the U.S. Army also tested the M80 7.62x51mm round fired from an M14 to compare it to the performance of the 5.56mm in CQB conditions. It performed in the same band of performance as the 5.56mm ammo tested. They concluded that in a CQB situation the 7.62x51mm round
offered no measurable performance benefit over the 5.56mm round.
One more important note, they also concluded that "shot placement trumps all other variables". This is something I've been saying for years (and in some of my videos found on my YouTube channel). Take the weapon you can best hit the target with, then worry about what caliber it is.
Source :
http://wstiac.alionscience.com/pdf/WQV8N1_ART01.pdf
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Mk 262 77-grain 5.56mm ammunition
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.
Two versions of the round have been procured to date. Initial production runs, designated Mark 262 Mod 0, lacked a cannelure. Subsequent production, designated Mk 262 Mod 1, added a cannelure to the bullet for effective crimping.
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" at more than 300 meters. Apparently it is superior to the standard M855 round when fired from an M4 or M16 rifle.
It evidently 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 AR15 platform weapon chambered to .223/5.56 mm.
Superior accuracy, wounding capacity, stopping power and range power has made this the preferred round of many Special Forces operators..
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There is a new 5.56 INSAS ammunition that has been approved but not inducted by the army. Probably higher velocity and weight than the current 64 grain ammunition in use.
http://164.100.47.134/lsscommittee/Defence/16_Defence_3.pdf
I am posting the relevant parts from the report in full.