[5] Robert Hutton (technical editor of Guns & Ammo magazine[clarification needed]) started development of a powder load to reach the 3,300 ft/s (1,006 m/s) goal.

The SS109 used a 62 gr full metal jacket bullet with a seven grain mild steel tip to move the center of gravity rearward, increasing flight stability and thereby the chances of striking the target tip-first at longer ranges, in part to meet a requirement that the bullet be able to penetrate through one side of a WWII U.S. M1 helmet at 500 yd (457 m) (which was also the requirement for the 7.62×51mm NATO).

It had a slightly lower muzzle velocity but better long-range performance due to higher sectional density and a superior drag coefficient.

[34] According to Nammo, a Finnish-Norwegian ammunition producer, the 5.56×45mm NATO M995 armour piercing cartridge can penetrate up to 12 mm (0.47 in) of RHA steel at 100 meters.

[39] If 5.56 mm bullets fail to upset (yaw, fragment, or deform) within tissue, the results are less significant wounds that may not cause adequate blood loss or damage to immediately stop the target's attack or advances.

As expected, with decreased wounding effects, rapid incapacitation is unlikely: enemy soldiers may continue to pose a threat to friendly forces and violent suspects can remain a danger to law enforcement personnel and the public.

Two other yaw issues: Angle-of-Attack (AOA) variations between different projectiles, even within the same lot of ammo, as well as Fleet Yaw variations between different rifles, were elucidated in 2006 by the Joint Service Wound Ballistic Integrated Product Team (JSWB-IPT), which included experts from the military law enforcement user community, trauma surgeons, aero ballisticians, weapon and munitions engineers, and other scientific specialists.

Unless the gas port can be regulated or adjusted for higher pressures, suppressors for short barreled 5.56×45mm NATO firearms must be larger and heavier than models for standard length rifles to function reliably.

[47] Originally designed for use in the Mk 12 SPR, the ammunition has found favor with special forces[48] units who were seeking a more effective cartridge to fire from their M4A1 carbines.

[49] It has more consistent performance because it is not yaw-dependent like the M855; the nose fragments upon impact and solid rear penetrator continues to move relatively straight.

One possible danger is that it generates much greater pressure in the chamber when fired, decreasing service life of parts and increasing the risk of catastrophic failure of the weapon (though this has yet to occur).

[53] If the 5.56 mm bullet is moving too slowly to reliably yaw, expand, or fragment on impact, the wound size and potential to incapacitate a person is greatly reduced.

The cartridge cases must maintain all performance requirements when fully assembled, be able to be used by the Lake City Army Ammunition Plant, must be manufactured in quantities totaling approximately 45 million per year.

[69][70] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56×45mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal", except the Mini-14 "Target" model, which only fires .223), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56×45mm NATO ammunition.

Hit probability refers to the ability of a soldier to concentrate on firing in spite of their weapon's recoil and noise, which is noticeably different between the two cartridges.

[52] The SS109 was made to pierce steel helmets at long range from the Minimi, not improve terminal performance on soft tissue from rifles or carbines.

The factors of impact angle and velocity, instability distance, and penetration before yaw reduce the round's predictable effectiveness considerably in combat situations.

[126] On 24 June 2010, the United States Army announced it began shipping its new 5.56 mm cartridge, the M855A1 Enhanced Performance Round (EPR), to active combat zones.

The Enhanced Performance Round was made to nearly match the trajectory of the M855 to aid in training consistency—the SS109/M855 ballistic coefficient (G7 BC) of 0.151 was improved to 0.152 for the M855A1[143]—but the ranges to get desired effects are greatly extended.

The shooter for the Army was Rob Harbison, a contractor supporting small caliber ammunition capability development at Fort Benning Georgia.

Harbison was happy with the performance of the EPR, with his scores showing that the Army's newest general purpose round is accurate enough to go toe-to-toe in the competition with the best ammo that can be bought or hand-loaded.

[62][152] During Army carbine testing, the round caused "accelerated bolt wear" from higher chamber pressure and increased bore temperatures.

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.

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.

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.

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.

[49][157] Officially designated the Mk318 MOD 0 "Cartridge, Caliber 5.56mm Ball, Carbine, Barrier", and called SOST (Special Operations Science and Technology) ammunition, the 62-grain bullet fragments consistently, even out of a 10.5 in barrel.

The reverse drawn technique leaves an open tip as a byproduct of the manufacturing process, and is not specifically designed for expansion or to affect terminal ballistics.

They found that by replacing the lead with copper and slightly stretching the jacket around to crimp the nose even more, the bullet's ballistic coefficient increased.

To avoid visual confusion with the Mk 262 round, the bullet was entirely nickel-plated for a silver color; the enhanced silver-colored copper jacketed, open tip match, 62-grain projectile was named the Mk318 MOD 1.