Black Hills 5.56mm 77 grain OTM (Military designation Mk 262 Mod 1)
Developed as a specialty cartridge for use in military SPR sniper rifles, the Mk 262 Mod 1 from Black Hills Ammunition has proven significantly more capable, both in accuracy and terminal performance (damage to target), than previous 5.56mm military ammunition. The increase in accuracy is due mostly to Black Hills Ammunition's outstanding quality and consistency, but the reason for the increased terminal performance is a little more involved.
To understand what makes the Mk 262 Mod 1 round so effective, we first need to look at how bullets in general create damage on impact, and then understand what makes the Black Hills 5.56mm 77 grain rounds different.
Full Metal Jacket: Full metal jacket (FMJ) bullets are usually considered “practice ammo” because FMJ bullets are cheap and while great for punching paper, they don't do a whole lot of damage to living targets. That's not to say you'd want to be shot with one, but FMJ rounds tend to poke a pretty narrow hole through their target. That narrow hole can still be deadly, but it's a lot more survivable than being shot with some of the other bullet designs we're going to talk about.
Hollow Point or Soft Point: Hollow point bullets and soft point bullets both expand or “mushroom” at the front upon striking a soft target like an animal or human. This expansion creates a much larger leading surface area than FMJ bullets, and causes significantly more damage to the target. If, while expanding, parts of the leading edge fragment off, the bullet's surface area becomes reduced again, so many people consider a bullet design that allows fragments to break off a poor design.
Fragmenting Bullets: Contrary to popular belief, fragments breaking off of a bullet is not a bad thing. In fact, it's the most devastating wounding mechanism discovered to date. The key point that the “fragmenting is bad” crowd is missing is that those fragments don't just leave the bullet and stop; they leave the bullet, angling away, tearing their own path through tissue that would otherwise remain undamaged by the main projectile. The more the bullet fragments, the more paths are torn through tissue, causing rapid incapacitation of the target. The only valid claim against fragmenting bullet designs, is that the wound channel ends up not being as deep as bullets that remain intact, but those who consider this a problem have either a poor understanding of animal/human anatomy or a poor understanding of the importance of shot placement. More info here. Bullets can be designed to fragment by making them with a deep hollow point and a thin copper jacket or by a thin jacket alone.
The modified 77 grain Sierra MatchKing (SMK) bullet that Black Hills Ammunition loads into the Mk 262 Mod 1 cartridge, like most military 5.56mm bullets, is a fragmenting bullet. In fact, the US military required the manufacturer of the bullet to put a cannelure (light crimp) in the bullet to make it break and fragment easier. The base of the SMK bullet is heavier than the tip, so upon striking a target, inertia takes over and the base tries to overtake the tip, causing the bullet to start to tumble. At roughly one inch long, the bullet experiences tremendous stress when it becomes sideways in this tumble, and breaks in half. Both damaged halves of the bullet then shed additional fragments for a total of dozens of separate wound channels.
While most military 5.56mm bullets are designed to fragment, the extra weight (77 grains vs 55 or 62 grains) of the SMK allows for more potential fragments, and the extra length causes the bullet to break and fragment at lower velocities. This last point is important, and largely responsible for the widespread popularity of the Mk 262 Mod 1 round. Soldiers with shorter barreled guns like the M4 carbine and MK 18 CQBR like the increased accuracy of the round, but they especially benefit from the bullet's ability to cause damage at the lower velocities produced by those guns.
Speaking of velocity, the Mk 262 round is a true 5.56mm load which yields a roughly 200 fps velocity increase over similar .223 pressure loads. If your gun is chambered for 5.56mm NATO rounds, this 5.56mm 77 grain Black Hills load is hard to beat.