When shooters debate calibers for AR-15 builds, the two heavyweights that always rise to the top are 5.56 NATO and .300 AAC Blackout. Both run on the same magazines, same AR-15 lowers, and same bolt carrier groups, yet they behave almost like two completely different platforms when tested side by side.

And now in 2025, with the explosion of 5” PDW barrels, ultra-compact AR pistols, and backpack-gun builds, the question has shifted again: Which caliber performs better in extremely short barrels — 5.56 or 300 Blackout?

This expanded guide answers that question in detail and gives you the most complete, updated caliber comparison available today.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

5.56x45mm NATO is the standard military and NATO round. It’s known for:

• High velocity

• Low recoil

• Wide ammo availability

• Excellent long-range performance

• Low cost per round

Typical bullet weights range from 55gr to 77gr. The round performs best when it has enough barrel length to reach fragmentation velocity—usually 11.5” and longer, though many shooters run it in shorter builds.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

.300 Blackout was designed for:

• Short barrels

• Suppressed shooting

• Subsonic performance

• Heavier bullets (110–220gr)

• Superior terminal energy at close range

It drastically outperforms 5.56 inside 150 yards and is the preferred caliber for:

• SBRs

• Home defense

• Tactical entry guns

• Hog & deer hunting

• Ultra-compact AR builds

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

The biggest trend in the firearms world right now is the emergence of 5-inch AR barrels, especially in:

• AR pistols

• PDWs

• Backpack guns

• Truck guns

• Braced or folding-stock builds

• Suppressor-first configurations

A 5” barrel changes everything.
Calibers behave very differently when you cut barrel length this drastically.

Here’s how each caliber reacts:

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

Let’s be blunt: 5.56 was never intended for 5-inch barrels.

Major issues with 5” 5.56:

• Catastrophic velocity loss

• Massive fireball and concussion

• Reduced fragmentation, meaning less effective terminal ballistics

• Loud — very loud

• Short dwell time → cycling issues if not tuned

• Harsh on suppressors

Even premium 5.56 ammo struggles to reach its designed performance when fired from a 5” barrel. Typical real-world velocities from a 5” tube:

• 55gr: ~1,700–1,900 fps

• 62gr: ~1,600–1,800 fps

• 77gr OTM: ~1,500–1,700 fps

This is barely above pistol velocities, meaning 5.56 becomes more like a .22 Magnum with attitude. Effective range is typically under 100 yards, and terminal effect is unpredictable.

The one reason shooters still use 5” 5.56:

It’s compact and shares ammo with their other ARs. To accommodate, shop Moriarti's 5" 5.56 uppers now.
But performance-wise, 5.56 is not the ideal 5” barrel caliber.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

This is where .300 BLK dominates like a monster.

Why .300 BLK shines in 5” barrels:

• Designed for short barrels — loses much less velocity

• Powder burns efficiently in 5–10"

• Heavy bullets still deliver huge energy

• Supersonic loads stay effective

• Subsonic loads are nearly silent when suppressed

• No fragmentation dependency — rounds expand or punch deep

Typical real-world 5” velocities:

• 110gr VMAX: ~2,000 fps

• 125gr SST: ~1,900 fps

• 150gr FMJ: ~1,600 fps

• 220gr subsonic: ~900–1,050 fps

This is outstanding performance from such a short barrel.
Even with only 5”, .300 BLK remains reliable, accurate, and terminally effective.

In short:

When you cut a barrel down to 5 inches, .300 Blackout becomes one of the best AR calibers on Earth.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

Dependent on velocity.
If the bullet doesn’t fragment, performance suffers.
Short barrels remove most fragmentation capability.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• Heavy bullets expand or penetrate reliably

• Much more energy delivered at CQB distances

• Even subsonic plays at a whole different level

• Performs extremely well even from 5–10" barrels

For home defense or close-range protection, 300 BLK is significantly more effective.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

This cannot be overstated:

Suppressed 5.56

• Still loud — the supersonic crack remains

• Harsh gas blowback

• Extreme flash in 5” barrels

• Hard on suppressors

Suppressed .300 BLK

• Subsonic = movie-quiet

• Comfortable indoors

• Minimal gas blowback

• Designed specifically for this role

• Works beautifully even in 5” barrels

If you’re suppressing a 5-inch AR, .300 BLK isn’t just better — it’s in a different league.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• Low recoil

• Common ammo

• Good with premium soft points

But inside a house:

• Extremely loud

• Over-penetration varies

• Flash and concussion are severe in short barrels

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• Massive stopping power

• Subsonic + suppressor = ideal for indoor CQB

• Less muzzle flash

• More reliable penetration control with expanding bullets

A suppressed 5” .300 BLK is arguably the most effective home-defense AR rifle setup available today.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• Coyotes

• Varmints

• Medium range shooting

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• Hogs

• Whitetail deer (inside 200 yds)

• Brush environments

• Forest, swamp, and thick cover

For most U.S. hunting distances, .300 BLK wins.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• 5.56 = light recoil

• 300 BLK supersonic = slightly more

• 300 BLK subsonic = extremely soft

5” rifles recoil a touch more due to the short impulse, but both remain very manageable.

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

• 5.56 = cheapest, most available

• 300 BLK = more expensive but increasingly common due to SBR/PDW popularity

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

Choose 5.56 If You Want:

• Cheapest ammo

• Longer range

• Traditional 16" rifle performance

• Training, plinking, and high-volume shooting

Choose .300 BLK If You Want:

• Best performance in 5” barrels

• Home defense optimization

• Suppressed shooting

• Heavy punch at short to medium distances

• Tactical/SBR/PDW builds

• Hunting hogs or deer at realistic ranges

5.56 vs .300 Blackout: The Ultimate 2025 Comparison

For 16” rifles and longer:
5.56 is still king for range, affordability, and versatility.

For anything 10” or shorter — especially 5" builds: .300 Blackout absolutely dominates.

This is why the 2025 AR market is shifting rapidly toward ultra-compact 300 BLK rifles and uppers.

And because the AR platform is modular, you can own both simply by swapping uppers — like the Moriarti 5" complete upper receiver in various configuration, or a short Moriarti 300 BLK 5” PDW Upper for suppressed CQB.