Two Legendary Rounds-300 AAC Blackout vs. 7.62x39mm

For most shooters, the difference between .300 AAC Blackout and 7.62x39mm boils down to what's cheaper at the gun counter. But for precision rifle builders, reloaders, and ballistic specialists, the comparison runs much deeper.

300BLKOUT(left) 7.62X39MM(right)

At Redleg, we build rifles for a specific purpose—whether that’s suppressed short-barreled hunting rigs or rugged semi-auto carbines. This article compares the .300 BLK and 7.62x39mm with technical depth across platform compatibility, pressure curves, barrel harmonics, projectile performance, and reloading intricacies. If you’re designing, building, or tuning high-performance rifles, here’s what actually matters.

Cartridge Origins & Intent

7.62x39mm: Legacy Warfighter Round

Designed in 1943 by the Soviets, 7.62x39mm was optimized for intermediate combat distances from stamped-sheet-metal, gas-piston rifles. The case geometry is heavily tapered to ensure reliable extraction in harsh field conditions. Its operating pressure is modest (typically ~45,000 PSI), and it favors reliability over precision.

• Bullet Diameter: .311” (note: U.S. barrels are often cut at .310” or even .308” to accommodate ammo variability)

• Case Length: 38.7mm (1.524")

• Case Taper: 0.045" from base to shoulder

• Standard Chamber Pressure: ~45,000 PSI

.300 AAC Blackout: Suppressed, Compact Performance

The .300 BLK was designed from the ground up for the AR-15 platform. It maintains the 5.56 NATO bolt face, feeds from standard magazines, and delivers .30-caliber ballistics in a package ideal for SBRs and suppressors. It’s one of the few rounds optimized around both subsonic and supersonic performance, depending on mission or environment.

• Bullet Diameter: .308”

• Case Length: 34.7mm (1.368")

• Case Taper: Minimal (designed for mag reliability in STANAG mags)

• Standard Chamber Pressure: ~55,000 PSI

Case Geometry & Chamber Design

Here’s where rifle builders must pay attention.

Tapered Case: 7.62x39mm

The 7.62x39mm’s steep case taper makes reliable feeding a challenge in non-curved magazines and AR-platform lowers. Even with optimized followers (e.g., ASC or C Products mags), consistent feeding can be finicky—especially in colder climates where lubricants thicken. The bolt face, extractor geometry, and feed ramp angle must be tuned carefully to ensure reliability.

Also, beware headspace drift in high-volume guns. The steel-cased nature of most surplus ammo doesn’t obturate well, increasing carbon fouling and gas blowback.

Straight-Walled, Short-Case: .300 Blackout

The .300 BLK case has minimal taper and was engineered to feed reliably from existing 5.56mm STANAG magazines. This gives it a major logistical advantage for builders using forged or billet AR receivers. No magazine curve to manage, and feeding is consistent across bullet weights—from 110-grain supersonics to 220-grain subs.

The shorter case also promotes faster cycling and easier suppression tuning in gas systems under 10”.

Internal Ballistics & Pressure Curves

This is critical for builders and suppressor integrators.

.300 BLK: Pressure Behind the Bullet

Subsonic .300 BLK loads (e.g., 220-gr SMK) create long, smooth pressure curves with low gas volume. Combined with short barrels (~9”), this makes for exceptional suppressor compatibility. It’s quiet, with minimal bolt velocity and low port pressure.

• Standard Port Pressure: ~11,000–14,000 PSI

• Optimal dwell time: Short gas systems with ~7.5” barrels for subsonics, ~9”-10.5” for supers

• Common Twist Rate: 1:7 or faster to stabilize 200–240 grain projectiles

Supersonic loads (110–135gr) push muzzle velocities of 2,200–2,300 fps from a 16” barrel, ideal for medium game hunting. These lighter loads benefit from slower twist rates (1:8–1:9) if accuracy at distance is a priority.

7.62x39mm: Abrupt Gas Curve

The x39 cartridge generates higher bolt thrust at lower chamber pressure, due to its larger base diameter and case taper. The internal pressure rises quickly and dumps early, which can make suppressor tuning more violent and bolt velocity harder to control—especially in direct-impingement ARs.

• Standard Port Pressure: ~16,000–18,000 PSI

• Barrel Length Efficiency: Gains velocity per inch more slowly than .300 BLK; optimal in 16–20”

• Common Twist Rate: 1:9.5 or 1:10, depending on barrel maker

Suppressing 7.62x39mm can be loud and dirty due to powder type (often fast-burning and high-volume), and subsonic factory loads are limited and unreliable in cycling DI systems.

Projectile Profiles & Terminal Ballistics

.300 BLK Supersonic

Lighter .30-cal bullets like the 110gr V-MAX or Barnes 110gr TAC-TX expand rapidly on soft targets and are ideal for deer, hogs, and coyotes. With quality projectiles and good velocity, they outperform traditional 7.62x39mm soft points at equivalent ranges, especially under 200 yards.

Subsonic performance is another story. A 220-grain SMK at 1,030 fps won’t expand, but it penetrates deeply and remains stable in tissue. With monolithic subsonic expanding bullets (like the Lehigh Defense Controlled Fracturing series), it becomes a viable terminal threat—but accuracy is load-dependent and trajectory is steep.

7.62x39mm

The 123gr M43 FMJ is infamous for over-penetration and tumbling yaw. While the round produces decent terminal effects, expansion is often unpredictable without modern hunting loads. Soft point commercial ammo (e.g., Hornady SST) has improved its terminal performance, but it's not in the same class as Blackout’s modern bonded and monolithic bullets.

Ballistically, x39 retains velocity a bit longer due to its lighter projectiles and higher starting velocity in 16”+ barrels. At 200+ yards, drop and drift are marginally better than subs, but far behind Blackout supers in precision rifles.

Accuracy Potential

.300 BLK

Capable of sub-MOA in custom builds using match-grade barrels, precision-cut chambers (e.g., PTG .300 BLK Match reamers), and proper twist for the intended bullet weight. Chamber concentricity and gas port placement are critical. In bolt rifles, subsonic loads can yield ~1.25–1.5 MOA with SMKs and better with optimized subsonic bullets.

7.62x39mm

Most commercial barrels have larger bores (.310–.311), and surplus ammo runs wide in tolerances. Even with handloads and good barrels (e.g., Criterion or Faxon), best-case accuracy hovers around 1.5–2 MOA. The inconsistency of surplus brass, steel cases, and wide bullet tolerances limit precision potential unless you build your own brass and bullets.

Reloading Considerations

.300 BLK

• Formed easily from .223/5.56 brass

• Trim length is critical; 1.368" max

• Neck tension must be carefully managed, especially with long subsonics

• Powder selection for subsonics: A1680, H4198, CFE BLK

• Crimping often required for heavy projectiles to prevent setback

7.62x39mm

• Brass cases available but not as common

• Steel cases not reloadable

• Bullets must be sized correctly: .310 or .311 depending on barrel

• Powder options: H335, CFE BLK, IMR 4198

• Case taper affects seating and crimp uniformity

Final Verdict: Builder’s Perspective

Feature	.300 Blackout	7.62x39mm
Suppressed Performance	⭐⭐⭐⭐⭐	⭐⭐
AR Platform Integration	⭐⭐⭐⭐⭐	⭐⭐
Accuracy Potential	⭐⭐⭐⭐	⭐⭐
Hunting Utility	⭐⭐⭐⭐ (supers)	⭐⭐⭐
Ammo Cost	⭐⭐	⭐⭐⭐⭐⭐
Reloading Flexibility	⭐⭐⭐⭐	⭐⭐
Parts Availability	⭐⭐⭐⭐⭐	⭐⭐
Legacy/Global Use	⭐⭐	⭐⭐⭐⭐⭐

Closing Thoughts

Both cartridges deliver .30-caliber performance in a compact package, but their applications and design philosophies are fundamentally different. If you're building a suppressed SBR or tuning for subsonic precision, .300 Blackout offers unmatched control. If your priority is rugged simplicity, ammo stockpiling, or cost efficiency, 7.62x39mm still holds value.

At Redleg, we build both—but we tailor the platform based on what you want the rifle to do. Whether it’s a suppressed bolt-action for nighttime hog hunts or a precision AR upper that runs subs like a sewing machine, we’ll spec the build from chamber to crown.

Need help deciding or designing?Let’s build it right the first time.

Contact Redleg or call us at (507) 677-6007.