The quick comparison
| Factor | 5.56 NATO | .300 Blackout |
|---|---|---|
| Cost per round (FMJ) | $0.30–0.42 | $0.50–0.80 |
| Cost per round (defense) | $0.50–1.00 | $0.70–1.50 |
| Suppressor performance | Decent — still supersonic | Excellent — true subsonic option |
| Short barrel performance | Drops off below 10.5" | Optimized for 9" barrels |
| Over-penetration (supersonic) | Low — fragments readily | Moderate — heavier bullets penetrate more |
| Over-penetration (subsonic) | N/A | Higher — 190gr+ bullets don't fragment |
| Magazine compatibility | Standard AR-15 | Standard AR-15 (same lower, different upper) |
| Ammo availability | Excellent — everywhere | Good — most major retailers |
When 5.56 wins
Cost. This is the biggest practical difference. 5.56 training ammo costs roughly 40–50% less than .300 Blackout. Over thousands of rounds of practice, that's significant. If your home defense plan includes regular training (it should), the cost advantage of 5.56 matters.
Fragmentation / reduced over-penetration. Lightweight, fast 5.56 bullets (55gr M193 at 3,100+ fps) fragment on impact with soft tissue and shed energy quickly through drywall. This makes 5.56 one of the better choices for interior defense — counterintuitive but well-documented in testing.
Barrel length flexibility. 5.56 works well from 10.5"–20" barrels. Most home defense ARs run 14.5"–16", which is 5.56's sweet spot.
Availability. 5.56 NATO is the most widely available centerfire rifle cartridge in America. During shortages, you'll find 5.56 long after .300 BLK shelves are empty.
When .300 Blackout wins
Suppressor use. .300 BLK was purpose-built for this. Subsonic .300 BLK (190–220gr at ~1,050 fps) through a suppressor is dramatically quieter than suppressed 5.56. If you're running a suppressed home defense rifle, .300 BLK subsonic is the clear winner for hearing protection and neighbor relations.
Short barrels. .300 BLK was designed for 9" barrels. Unlike 5.56, which loses significant velocity and terminal performance from barrels under 10.5", .300 BLK achieves full ballistic potential from a 9" barrel. If you want the shortest possible home defense rifle (SBR or pistol-brace configuration), .300 BLK has the advantage.
Heavy barriers. Supersonic .300 BLK (110–125gr at 2,200–2,400 fps depending on barrel length) punches through intermediate barriers (car doors, heavy furniture) better than lightweight 5.56 that may fragment prematurely.
The subsonic tradeoff
Subsonic .300 BLK (the main selling point for suppressor use) has a significant home defense limitation: heavy, slow bullets don't fragment or expand reliably. A 190gr subsonic round behaves more like a pistol bullet — it punches through walls and keeps going. This increases over-penetration risk compared to supersonic 5.56.
If over-penetration is your primary concern (apartment, thin walls, family members in adjacent rooms), supersonic 5.56 is actually safer than subsonic .300 BLK.
The counter-argument: if you fire an unsuppressed 5.56 rifle indoors without hearing protection, you may be temporarily deafened after the first shot. A suppressed .300 BLK subsonic round is hearing-safe, allowing you to maintain situational awareness. Both factors matter in a real home defense scenario.
Cost reality
For identical training volume, .300 BLK costs roughly 60–100% more than 5.56:
| Annual training | 5.56 cost | .300 BLK cost | Difference |
|---|---|---|---|
| 1,000 rounds | $320 | $580 | +$260 |
| 2,500 rounds | $800 | $1,450 | +$650 |
| 5,000 rounds | $1,600 | $2,900 | +$1,300 |
The dangerous mistake
.300 Blackout and 5.56 NATO use the same magazines, the same lower receiver, and load identically. A .300 BLK round will chamber in a 5.56 barrel but will not clear the bore. Firing a .300 BLK round in a 5.56 barrel causes a catastrophic failure — destroyed upper receiver, possible injury.
If you own rifles in both calibers, label your magazines clearly and store ammo separately. This is the most dangerous aspect of .300 BLK ownership and has caused multiple documented explosions.
The bottom line
Choose 5.56 if: Cost matters, you don't have a suppressor, your barrel is 10.5"+, and over-penetration through interior walls is your primary concern.
Choose .300 BLK if: You run a suppressed SBR, you want the quietest possible home defense setup, and you accept the higher cost and increased wall penetration risk of subsonic loads.
For most people, 5.56 is the better home defense choice. It's cheaper to train with, fragments to reduce over-penetration, and doesn't require a suppressor to reach its potential. .300 BLK is the specialist's choice — excellent at what it does, but with tradeoffs that not everyone needs to accept.
Search 5.56 ammo → | Search .300 BLK ammo →
Related articles
Related caliber pages
- .300 Blackout ammo prices — Current pricing across all retailers
- Cheapest .300 Blackout right now — Lowest observed prices
- .300 Blackout price history — 30-day pricing trends
- 5.56 NATO ammo prices — Current pricing across all retailers
- Cheapest 5.56 NATO right now — Lowest observed prices
- 5.56 NATO price history — 30-day pricing trends
Sources
- .300 AAC Blackout — Wikipedia — Cartridge development and specifications
- SAAMI .300 Blackout Specifications — Official pressure standards
- Hornady .300 Blackout Products — Subsonic and supersonic options