500 round Case Black Hills Ammunition Factory New D556N21 - 5.56 NATO 70 Grain Hornady CX in 50 round boxes

While Black Hills is globally famous for its match-grade open-tip projectiles (like the MK 262), the D556N21 represents their absolute pinnacle in monolithic, barrier-blind, hunting, and defensive performance. This cartridge was specifically introduced to replace older configurations featuring the Hornady GMX bullet, utilizing Hornady’s more aerodynamically advanced CX projectile.

Overview & Technical Specifications

The core of this load is the Hornady CX (Copper alloy eXpanding) bullet. Unlike traditional jacketed lead bullets, the CX is a single-piece, solid monolithic copper alloy wheelhouse. It features a polymer tip to kickstart expansion and progressive grooves cut into the driving band to reduce fouling and pressure.

• Caliber: 5.56x45mm NATO (Full military pressure; do not fire in a standard .223 Remington chamber)

• Bullet Weight: 70 grain (Heavy for a monolithic bullet)

• Bullet Type: Hornady CX Monolithic Solid Copper Alloy Hollow Point with a polymer tip

• Casing: Brass, boxer-primed, reloadable

• Muzzle Velocity: ~2,800 to 2,850 ft/s (depending on barrel length)

• Muzzle Energy: ~1,230 ft-lbs

Performance and Ballistics

1. Hardcore "Barrier Blindness"

The primary reason to buy the D556N21 is its structural integrity. Traditional lead-core hollow points can shear, fragment, or completely separate when they hit tough obstacles like automotive glass, heavy sheet metal, or thick plywood. Because the Hornady CX bullet is a solid piece of copper alloy, it easily punches through these barriers while retaining 95% to 100% of its total weight. Once it clears the barrier, it still penetrates and expands predictably.

2. Terminal Expansion & Hunting Capability

Upon hitting soft tissue (or game), the polymer tip is forced backward into the hollow cavity, causing the copper nose to peel open into a wide, uniform four-petal mushroom.

Because copper is lighter than lead, a 70-grain copper bullet is physically quite long—roughly the same physical length as a 77-grain lead bullet. This long profile, combined with zero weight fragmentation, results in straight, deep penetration (often 16 to 20+ inches in ballistic gelatin). This makes it highly effective for medium-sized game like deer and hogs, as well as an elite defensive round.

3. Twist Rate Demands

Because this 70-grain solid copper bullet is exceptionally long, it requires a fast twist rate to stabilize properly.

• 1:7" Twist: Optimal. It will stabilize the bullet perfectly for maximum precision.

• 1:8" Twist: Fully acceptable and highly accurate.

• 1:9" Twist or slower: Avoid. A 1:9 twist rifle will struggle to stabilize a bullet this long, which can result in tumbling and terrible accuracy.

Pros & Cons

Pros

• Near-Perfect Weight Retention: Will not break apart when encountering harsh barriers or heavy bone.

• Premium Black Hills Loading: Exceptionally low standard deviations in velocity, consistent powder charges, and match-grade brass prep.

• 100% Lead-Free: Fully California-compliant and legal for hunting in areas with strict non-toxic ammunition mandates.

• Cleaner Running: Hornady’s "Optimum Groove Geometry" on the bullet body actively reduces copper fouling in the barrel compared to older solid-copper designs.

Cons

• High Cost: Monolithic copper bullets are expensive to machine. Expect to pay premium prices per box, making it strictly a "duty, hunting, or defense" round.

• Reduced Fragmentation: If you are looking for a round that violently fragments into dozens of tiny pieces inside a target (like an OTM match round), this isn't it. The CX is built to stay together and penetrate deeply.

• Strict Chamber Requirements: It is heavily loaded to true 5.56 NATO pressures. It should never be used in older bolt-actions or rifles stamped exclusively for .223 Rem.

The Verdict

The Black Hills D556N21 (70gr Hornady CX) is an elite, specialized round. If you are looking for match-grade target ammunition, stick with Black Hills' MK 262 (77gr). But if you need an ultra-reliable, barrier-blind defensive round, or a premier 5.56mm hunting load for medium game that will break through heavy bone and track deeply, this monolithic monster is as good as it gets.