What Caliber Conversions Can I Do on an AR-15?
The AR-15 platform supports dozens of caliber conversions. The easiest require only a new upper (barrel, BCG, and sometimes magazine): .300 Blackout (same bolt and magazine as 5.56), 6.5 Grendel (new bolt, new magazine), 6.8 SPC (new bolt, new magazine), .350 Legend (new barrel, same bolt), .450 Bushmaster (new bolt, new magazine), and .22 LR (dedicated conversion kit or upper). Your existing lower receiver works with all of these.
Why It Matters
One of the AR-15's greatest strengths is modularity. A single lower receiver can host multiple uppers in different calibers, giving you different capabilities without buying separate firearms. Understanding which conversions are drop-in, which require new bolts or magazines, and which require a different lower (AR-10) helps you plan a versatile collection efficiently.
Switching calibers means new ammo — find 5.56, 300 Blackout, 6.5 Grendel, and more.
Shop Rifle Ammo at Lucky Gunner →The Detail
AR-15 caliber conversions (standard AR-15 lower):
Drop-in easy (same bolt face as 5.56, same magazine):
- .300 Blackout: new barrel only, uses 5.56 bolt and magazines
- Supersonic: 110-125gr at 2,200 fps, comparable to 7.62x39
- Subsonic: 190-220gr at 1,050 fps, excellent suppressed
- DANGER: 300 BLK rounds fit in a 5.56 chamber — catastrophic failure if fired. Mark everything clearly.
- .350 Legend: new barrel, uses modified 5.56 bolt, works in some AR magazines
- Straight-wall cartridge, legal for deer in restricted states (Ohio, Michigan, Iowa, Indiana)
- 150-180gr at 2,100-2,300 fps
New bolt required (same magazine well):
- 6.5 Grendel: new barrel, new bolt, Grendel-specific magazines (modified AR mags)
- 123gr at 2,580 fps, excellent long-range performance to 800 yards
- Better ballistics than 5.56 at distance with more energy
- 6.8 SPC / 6.8 SPC II: new barrel, new bolt, 6.8-specific magazines
- 115gr at 2,625 fps, more energy than 5.56 at all ranges
- Developed by special operations for better terminal performance
- .224 Valkyrie: new barrel, new bolt, 6.8 SPC magazines
- 90gr at 2,700 fps, designed for long-range AR-15 shooting
- Excellent ballistic coefficient for wind resistance
Big bore (same lower, new upper and magazines):
- .450 Bushmaster: new barrel, new bolt, single-stack modified magazines
- 250gr at 2,200 fps, straight-wall deer cartridge
- Massive energy, significant recoil
- .458 SOCOM: new barrel, new bolt, uses modified 5.56 magazines (single stack)
- 300gr at 1,900 fps, subsonic capability
- CQB powerhouse, excellent suppressed with heavy subsonic loads
- .50 Beowulf: new barrel, new bolt, modified 5.56 magazines
- 350gr at 1,800 fps
- Maximum stopping power from AR-15 platform
Rimfire conversion:
- .22 LR: dedicated conversion bolt or complete upper
- CMMG .22 LR conversion kit ($160-$200): drops into existing 5.56 upper
- Dedicated .22 LR upper: more accurate, purpose-built
- Excellent for cheap practice ($.05-$.08/rd vs $.30+/rd for 5.56)
NOT compatible with AR-15 lower (requires AR-10/LR-308):
- .308 Winchester / 7.62x51 NATO
- 6.5 Creedmoor
- .243 Winchester
- .338 Federal
Build Impact
The most practical first conversion is .300 Blackout — buy a complete 300 BLK upper and swap it onto your existing 5.56 lower in seconds. Use the same magazines and bolt carrier group. Label everything clearly to prevent mixing ammunition. For cheap practice, a .22 LR conversion kit pays for itself in 1,000 rounds of ammo savings. For hunting in straight-wall-only states, .350 Legend or .450 Bushmaster are the top choices.