The comparison between .300 Blackout (300 BLK) and 5.56×45 NATO is one of the most practical debates for users of AR-pattern rifles and short-barrel rifles (SBRs). Both cartridges are commonly fired from the same platform family, yet they serve very different missions: 5.56 is an intermediate, velocity-driven cartridge optimized for lighter projectiles and longer effective range; .300 Blackout was designed to deliver heavy-bullet performance (including subsonic loads) from the same basic system, especially when suppressed or fired from short barrels.
This article offers a professional, in-depth comparison of ballistics, suppressor performance, barrel-length considerations, terminal effects, logistics, and recommended use cases so you can choose the right cartridge for your intended role.
Origins & Design Goals
5.56×45 NATO
5.56 NATO was developed to offer a lightweight, relatively low-recoil cartridge that allowed soldiers to carry more rounds and control automatic or rapid semiautomatic fire. It favors high velocity and fragmentation with light, fast projectiles and is optimized for engagements at typical small-arms combat distances.
.300 Blackout
.300 Blackout was designed specifically to give AR-pattern users access to heavier bullets—particularly subsonic projectiles—that perform well when suppressed and out of short barrels. The round achieves that by using a .30 caliber bullet seated in a modified 5.56/.223 case. The result is a cartridge that can use the same bolt, magazines, and most of the same components as 5.56, while offering a very different ballistic profile.
Basic Ballistics: Velocity, Bullet Weight, and Energy
Below is a general, representative comparison. Real numbers vary by manufacturer, barrel length, and load type.
Cartridge
Typical Bullet Weight
Muzzle Velocity (from ~16–16.5″ barrel typical for 300 BLK & 14.5–16″ for 5.56)
Primary Design Goal
5.56 NATO
55–77 grain
~2,700–3,100 fps
High velocity, flatter trajectory, fragmentation potential
.300 Blackout (supersonic)
110–150+ grain
~1,900–2,400 fps (varies widely by barrel length)
Enhanced energy on target; better short-range terminal performance
.300 Blackout (subsonic)
190–220 grain
~1,000 fps or less (designed to stay below sound barrier)
Maximum suppressed performance, reduced blast and sound signature
Key takeaway: 5.56 is a high-velocity cartridge with a flat trajectory; .300 Blackout trades velocity for heavier bullets and is tailored to suppressed and short-barrel use.
Suppressor (Silencer) Performance & Signature
Suppressor effectiveness depends heavily on whether the round is subsonic or supersonic. The sonic crack produced when a bullet travels faster than the speed of sound is often the dominant sound source; suppressors mitigate muzzle blast but cannot eliminate the sonic crack of a supersonic projectile.
5.56 + suppressor: Suppressing a supersonic 5.56 round still leaves a pronounced sonic crack. You get much reduced muzzle blast, but the transonic signature remains audible. Subsonic 5.56 loads exist but are uncommon and offer poor terminal performance.
.300 Blackout (subsonic) + suppressor: This is where 300 BLK shines. A heavy subsonic projectile (190–220 gr) traveling under ~1,050 fps eliminates the sonic crack and, when combined with a good suppressor, produces a very low report — much quieter than suppressed supersonic 5.56. This makes .300 Blackout the preferred option for suppressed short-range work.
.300 Blackout (supersonic) + suppressor: Still benefits from blast suppression, and the heavier bullets maintain good terminal performance, but the sonic crack will still be present.
For operations or hunting scenarios where minimizing auditory signature is a priority, subsonic .300 Blackout is the clear winner.
Barrel Length Effects and Short-Barrel Rifles (SBRs)
Barrel length dramatically impacts both cartridges—but in different ways.
5.56 & Short Barrels
5.56 is optimized for longer barrels; velocity (and thus terminal performance and effective range) falls off quickly in very short barrels (≤10.5–12.5″), reducing fragmentation and long-range effectiveness.
Shortened 5.56 barrels maintain utility for close-in use, but ballistics and terminal effects change significantly as velocity drops.
.300 Blackout & Short Barrels
.300 Blackout was designed to be efficient from short barrels. Supersonic loads still retain meaningful velocity from 8.3–10.5″ barrels, and subsonic loads are largely unaffected by barrel length in terms of staying subsonic (they’re intentionally slow).
That makes .300 Blackout ideal for SBRs and suppressed short-barrel configurations where reliability and performance from a compact package are priorities.
If your primary platform is a short, maneuverable rifle or SBR intended to be used suppressed, .300 Blackout will usually outperform 5.56 in terminal effect and sound signature.
Terminal Performance & Practical Effects
Terminal performance is complex: it depends on bullet design, velocity, construction, and what the target is. A few high-level comparisons:
5.56 at optimal velocity: Lighter bullets at high velocity can fragment and produce devastating wound profiles at typical engagement ranges. Fragmentation is velocity-dependent—drop below certain speeds and fragmentation stops, reducing that particular wounding mechanism.
.300 Blackout supersonic: Heavier bullets at supersonic speeds deliver more momentum and can provide deeper penetration and larger permanent wound channels at short range compared to a similarly situated 5.56 round from a short barrel.
.300 Blackout subsonic: Heavy, subsonic bullets provide strong momentum and penetration at close range and, with modern controlled-expansion designs, can still be effective in stopping roles while being much quieter when suppressed.
In short-range, suppressed, or short-barrel roles: .300 Blackout generally provides more reliable terminal performance than 5.56. For longer ranges where velocity and flat trajectory matter, 5.56 has the advantage—assuming sufficient barrel length.
Barrier interaction is important in defensive scenarios.
5.56 supersonic rounds may fragment on soft tissue but can be stopped or altered by barriers; barrier strikes may reduce lethality if fragmentation is prevented.
.300 Blackout supersonic rounds, with their heavier bullets, often penetrate barriers more predictably, retaining weight and energy through intermediate obstructions.
.300 Blackout subsonic heavy bullets can penetrate deeply and may over-penetrate in some scenarios; choosing the right bullet construction (bonded, controlled expansion designed for subsonic velocities) mitigates this risk.
For urban defensive contexts where over-penetration is a concern, thoughtful load selection and understanding of local environment become the critical decision factors more than cartridge alone.
Logistics, Ammo Availability, & Cost
Historically, 5.56 production volumes are far larger than .300 Blackout, making 5.56 cheaper and widely available. However, .300 Blackout has grown in popularity, and factory supersonic and subsonic loads are common from major manufacturers.
5.56 advantages: Cheaper per round, more variety in match, FMJ, and specialty loads; ubiquitous in surplus and military channels.
.300 Blackout considerations: Typically pricier than bulk 5.56, and subsonic match/expansion loads can be premium; however, buying in small quantities for specialized roles is common.
Both cartridges share major platform components (magazines, bolts, receivers), simplifying logistics if you operate mixed calibers on the same basic system—though conversions and dedicated barrels are required for safe interchange.
Reliability & Extraction
Both cartridges are reliable when using quality barrels and gas systems tuned to the load. A few notes:
Some supersonic .300 Blackout loads require gas system adjustments or specific buffer weights to tune cycling, particularly in short barrels.
Subsonic .300 Blackout often cycles more smoothly in integrally suppressed or properly tuned blowback/tuned gas systems, but many users prefer using a suppressor-friendly gas setting or a dedicated suppressed upper for best results.
5.56 in short barrels requires gas system tuning to ensure reliable cycling and extraction; in very short barrels (8–10.5″), dwell time and gas port sizing become critical.
Use Cases & Recommended Roles
Choose 5.56 NATO if you want:
Longer effective range and flatter trajectory for mid-range engagements
Lower recoil and easier follow-up shots at moderate distances
Lower cost ammo for high-volume training
High-capacity magazines and lighter overall kit weight
Choose .300 Blackout if you want:
Excellent suppressed performance, especially with subsonic loads
Superior short-range terminal effect in SBRs or suppressed configurations
Optimized performance from short barrels (8–16″), where 5.56 loses velocity fast
Flexible use of both supersonic and subsonic loads in the same barrel for mission adaptability
Practical Recommendations
For a primary patrol/defense or general-purpose rifle: 5.56 remains an excellent, economical choice with wide ammo availability and predictable ballistics out to several hundred yards.
For a compact suppressed SBR or a dedicated hunting/specialty suppressor host: .300 Blackout offers clear advantages, especially when subsonic performance is required.
For mixed fleets: Consider maintaining both calibers: 5.56 for open-area or training needs, and .300 Blackout for suppressed, short-range, or specialized roles.
Training guidance: Practice with the role-appropriate load. If you plan to run suppressed subsonic .300 Blackout primarily, get accompaniment practice with both subsonic and supersonic so you’re prepared for recoil and ballistic differences.
Final Evaluation
There is no universal “better” cartridge—only better fits for specific missions. 5.56 NATO is a highly effective, economical, and versatile intermediate cartridge that excels at moderate ranges when used from appropriately long barrels. .300 Blackout delivers purpose-built capability: heavy bullets, excellent suppressed subsonic performance, and efficiency from short barrels. For suppressed SBR work and compact suppressed hosts, .300 Blackout is often the better tool. For general-purpose duties, training, and longer-range precision inside its effective envelope, 5.56 remains the go-to choice.
Choose the cartridge that aligns with your platform, your expected engagement distances, the importance of suppression, and your training cadence. With thoughtful load selection and system tuning, either cartridge can perform exceptionally in the role it was intended to fill.
Always comply with local laws, follow safe handling practices, and verify compatibility before changing barrels or cartridges in any rifle platform.
