AR-15 Caliber Selection Guide 2025: 5.56 vs .300 BLK vs 6.5 Grendel vs 7.62x39 header image
AR-15 Caliber Comparison Table
CaliberMuzzle Velocity (16")Muzzle EnergyEffective RangeCost per RoundBest ForAvoid ForHot Take
5.56×45mm NATO / .223 Remington2900 fps1280 ft-lbs500 yards$0.35First AR build, General purpose carbine, Home defense, Training (cheap ammo), CompetitionDedicated suppressor host where subsonic matters, Hunting large game beyond 200 yards, Ultra-short barrels (under 10.5")If you only own one AR, this is the caliber. Anyone who tells you 5.56 is "obsolete" is selling something. The military hasn't replaced it because nothing else does the job better across all mission sets.
.300 AAC Blackout2200 fps1350 ft-lbs300 yards$0.85Dedicated suppressor host, SBR builds (8-10.5"), Home defense (suppressed), Hog hunting at close range, PDW / truck gunUnsuppressed general purpose (just use 5.56), Long-range shooting, Budget-conscious training, Mixed mag environments without strict color codingThis is THE suppressor cartridge for AR-15s. If you're not running a can, you're paying more for less performance than 5.56. But suppressed with subs? Nothing else comes close for an AR-pattern rifle.
6.5 Grendel2580 fps1580 ft-lbs800 yards$1.00Designated marksman rifle, Precision rifle competition, Hunting (deer, antelope, hogs), Long-range target shooting, Rural property defenseGeneral purpose carbine, SBR builds, High-volume training, Home defense, Budget-conscious shootersThe Grendel exists for one reason: to stretch the AR-15 to ranges it was never designed for. If you're not shooting past 500 yards regularly, you're paying a premium for capability you'll never use. But for DMR and hunting builds? It's the best option that fits an AR-15 lower.
7.62×39mm2350 fps1520 ft-lbs350 yards$0.35Budget-focused high-volume shooting, Dedicated range toy, Brush gun / hunting (with quality ammo), Shooters who reload and want .30 cal versatilityDuty/defensive use (reliability concerns), Precision shooting, First AR build, Anyone who doesn't want to troubleshoot magazine issuesIf you want 7.62x39, buy an AK. The AR-15 platform fights this cartridge at every turn—magazines suck, bolts break, and accuracy suffers. The only reason to run it is cheap steel-case ammo, and that advantage disappears with every import ban.
6mm ARC (Advanced Rifle Cartridge)2750 fps1650 ft-lbs700 yards$1.25Precision rifle competition, Designated marksman builds, Long-range hunting, Shooters who want the newest/bestGeneral purpose use, Budget builds, High-volume training, Anyone who needs ammunition availabilityThe 6mm ARC is what the Grendel should have been from the start. But it's a victim of timing—arriving when ammo supply chains are already strained. If Hornady can keep up production and others get licensed, this becomes the AR-15 DMR standard. Until then, it's for early adopters with deep pockets.
AR-15 Caliber Recommendations by Mission
MissionRecommended CaliberAlternativeReasoning
Home Defense5.56 NATO.300 BLK5.56 fragments reliably at indoor distances, reducing over-penetration risk. .300 BLK is the upgrade if you're running suppressed and want hearing-safe subsonic capability.
General Purpose Carbine5.56 NATO6.5 GrendelNothing beats 5.56 for general purpose—cheap ammo, universal parts, proven performance. Grendel if you need more range and are willing to pay the parts/ammo premium.
Suppressed SBR.300 BLK5.56 NATO.300 BLK was designed for exactly this mission. Subsonic performance is unmatched. 5.56 works suppressed but remains loud and loses significant velocity in short barrels.
Precision / DMR6mm ARC6.5 Grendel6mm ARC has the best ballistics for precision work if you can source ammo. 6.5 Grendel is the proven fallback with better ammunition availability.
Hunting6.5 Grendel.300 BLK6.5 Grendel offers ethical killing power on deer-sized game to 400+ yards. .300 BLK is excellent for hogs and whitetail inside 150 yards, especially suppressed.
Budget Training5.56 NATO7.62x395.56 steel-case runs around $0.30/round. 7.62x39 can be cheaper when imports flow, but magazine reliability issues make it less enjoyable for high-volume days.
Competition5.56 NATO6mm ARCMost practical rifle matches are built around 5.56 distances and power factors. 6mm ARC shines in PRS-style precision matches with longer range requirements.
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Reference guideCaliber selectionControversial

AR-15 Caliber Selection Guide

The caliber debate never ends. 5.56 purists argue with .300 BLK suppressor enthusiasts while Grendel fans shake their heads from 800 yards out. This guide cuts through the arguments with data, opinionated takes, and mission-specific recommendations so you can pick the right cartridge for your build.

Comparison of AR-15 cartridges: 5.56 NATO, .300 Blackout, and 6.5 Grendel
All ballistics from 16-18" barrels unless noted.Prices reflect late 2024 brass-case ammunition.Your mission dictates the answer—there is no universal "best."

Best General Purpose

5.56 NATO

Cheap, available everywhere, massive aftermarket. The cartridge the platform was designed around. Unless you have a specific reason to go elsewhere, start here.

Best Suppressed

.300 Blackout

Purpose-built for cans and short barrels. Subsonic loads are hearing-safe and effective. Same bolt and mags as 5.56—just swap the barrel.

Best Long Range

6.5 Grendel

Stretches the AR-15 past 800 yards with ethical hunting energy. Requires dedicated bolt and mags, but nothing else reaches this far on the platform.

The core controversy

5.56 vs .300 BLK is the holy war. Both fit standard AR-15 lowers. Both share bolts and magazines. The difference is philosophy: 5.56 maximizes velocity and range from longer barrels. .300 BLK maximizes suppressor performance and short-barrel efficiency.

The trap: People buy .300 BLK for "better performance" without owning a suppressor, then complain about $0.85/round ammo. If you're not running a can, 5.56 does everything .300 BLK does—cheaper and with better long-range ballistics.

The answer: 5.56 for your first AR. .300 BLK for your dedicated suppressor host. 6.5 Grendel for your precision upper. Different tools for different jobs.

>

Compare calibers head-to-head

Select up to 3 calibers to compare their ballistics, cost, and mission suitability. Filter by your intended use to see recommendations.

Select Calibers (max 3)

5.56 NATO

General purpose / baseline

Muzzle Velocity (16")2,900 fps ★
Muzzle Energy (16")1,280 ft-lbs
Supersonic Range800 yds ★
Effective Range500 yds ★
Cost per Round$0.35 ★
Availability10 /10 ★
Suppressor Friendly6 /10
Recoil Management9 /10
Hot Take

“If you only own one AR, this is the caliber. Anyone who tells you 5.56 is "obsolete" is selling something. The military hasn't replaced it because nothing else does the job better across all mission sets.”

.300 BLK

Suppressed SBR / subsonic specialist

Muzzle Velocity (16")2,200 fps
Muzzle Energy (16")1,350 ft-lbs ★
Supersonic Range450 yds
Effective Range300 yds
Cost per Round$0.85
Availability7 /10
Suppressor Friendly10 /10 ★
Recoil Management7 /10
Hot Take

“This is THE suppressor cartridge for AR-15s. If you're not running a can, you're paying more for less performance than 5.56. But suppressed with subs? Nothing else comes close for an AR-pattern rifle.”

Detailed caliber breakdown

Each cartridge has a specific role where it excels. Understanding the tradeoffs helps you avoid buying a solution to a problem you don't have.

5.56 NATO

The baseline. Decades of refinement, massive aftermarket, and proven terminal performance when velocity stays above threshold. The cartridge the AR-15 was designed around.

General purpose / baselineSBR: marginal
Velocity (16")
2,900 fps
Energy (16")
1,280 ft-lbs
Effective Range
500 yds
Cost per Round
$0.35
Availability10/10
Suppressor Friendly6/10
Recoil Management9/10

Pros

  • • Ubiquitous ammunition—every gun store, every sporting goods section, every online retailer.
  • • Massive aftermarket for barrels, uppers, and load data across every price tier.
  • • Proven terminal ballistics with quality expanding/fragmenting ammunition above 2500 fps.
  • • Low recoil enables fast follow-up shots and new-shooter friendliness.
  • • Standardized magazines, bolts, and parts—no compatibility headaches.

Cons

  • • Velocity-dependent terminal performance—short barrels (under 11.5") compromise effectiveness.
  • • Supersonic crack cannot be eliminated—poor for noise discipline even suppressed.
  • • Limited barrier penetration compared to larger intermediate calibers.
  • • Bullet selection matters significantly—cheap FMJ is poor for defensive use.

Best For

  • ✓ First AR build
  • ✓ General purpose carbine
  • ✓ Home defense
  • ✓ Training (cheap ammo)
  • ✓ Competition

Avoid For

  • ✗ Dedicated suppressor host where subsonic matters
  • ✗ Hunting large game beyond 200 yards
  • ✗ Ultra-short barrels (under 10.5")

Hot Take

“If you only own one AR, this is the caliber. Anyone who tells you 5.56 is "obsolete" is selling something. The military hasn't replaced it because nothing else does the job better across all mission sets.”

Build Notes

For barrels under 14.5", consider mid-length gas for reliability. Match barrel twist to bullet weight: 1:7 for 77gr, 1:8 for general purpose, 1:9 for 55gr varmint loads.

Optimal barrel: 14.5" - 16"

.300 BLK

Purpose-built for suppressed short barrels. Swaps only the barrel—same bolt, same mags. The only AR cartridge where subsonic is genuinely effective.

Suppressed SBR / subsonic specialistSBR: excellent
Velocity (16")
2,200 fps
Energy (16")
1,350 ft-lbs
Effective Range
300 yds
Cost per Round
$0.85
Availability7/10
Suppressor Friendly10/10
Recoil Management7/10

Pros

  • • Optimized for short barrels—loses minimal velocity below 10" compared to 5.56's dramatic drop.
  • • Same bolt and magazines as 5.56—barrel swap only for caliber conversion.
  • • Subsonic loads are hearing-safe suppressed and genuinely effective for defense.
  • • .30 caliber bullets offer better barrier penetration than 5.56.
  • • Burns powder efficiently in short barrels, reducing flash and concussion.

Cons

  • • Expensive ammunition—2-3x the cost of 5.56 for comparable quality.
  • • Supersonic loads are ballistically inferior to 5.56 past 200 yards.
  • • Magazine mix-ups with 5.56 can cause catastrophic failures (kaboom).
  • • Subsonic terminal performance requires quality expanding ammo—hardball is an ice pick.
  • • Limited long-range capability; trajectory drops fast.

Best For

  • ✓ Dedicated suppressor host
  • ✓ SBR builds (8-10.5")
  • ✓ Home defense (suppressed)
  • ✓ Hog hunting at close range
  • ✓ PDW / truck gun

Avoid For

  • ✗ Unsuppressed general purpose (just use 5.56)
  • ✗ Long-range shooting
  • ✗ Budget-conscious training
  • ✗ Mixed mag environments without strict color coding

Hot Take

“This is THE suppressor cartridge for AR-15s. If you're not running a can, you're paying more for less performance than 5.56. But suppressed with subs? Nothing else comes close for an AR-pattern rifle.”

Build Notes

Color-code all .300 BLK mags and uppers to prevent mixing with 5.56. A .300 BLK round chambers in a 5.56 barrel and will detonate on firing. Use pistol-length gas for 8-9" barrels, carbine for 10.5"+.

Optimal barrel: 8" - 10.5"

6.5 Grendel

The long-range AR-15 cartridge. High BC bullets maintain velocity downrange where 5.56 falls apart. Requires dedicated bolt and magazines.

Long-range precision / huntingSBR: poor
Velocity (16")
2,580 fps
Energy (16")
1,580 ft-lbs
Effective Range
800 yds
Cost per Round
$1.00
Availability5/10
Suppressor Friendly7/10
Recoil Management7/10

Pros

  • • Exceptional long-range performance from AR-15 platform—1000+ yard capable.
  • • High BC bullets retain energy and resist wind drift better than 5.56.
  • • Effective hunting cartridge for deer-sized game to 400+ yards.
  • • Stays supersonic past 1000 yards with quality match ammo.
  • • Standard AR-15 lower—only bolt and magazines change.

Cons

  • • Requires dedicated bolt face and magazines (not compatible with 5.56 parts).
  • • Expensive and less available ammunition—not a volume training round.
  • • Poor short-barrel performance—needs 18"+ to shine.
  • • Niche magazines can be finicky; stick with proven brands.
  • • Overkill for typical carbine distances (under 300 yards).

Best For

  • ✓ Designated marksman rifle
  • ✓ Precision rifle competition
  • ✓ Hunting (deer, antelope, hogs)
  • ✓ Long-range target shooting
  • ✓ Rural property defense

Avoid For

  • ✗ General purpose carbine
  • ✗ SBR builds
  • ✗ High-volume training
  • ✗ Home defense
  • ✗ Budget-conscious shooters

Hot Take

“The Grendel exists for one reason: to stretch the AR-15 to ranges it was never designed for. If you're not shooting past 500 yards regularly, you're paying a premium for capability you'll never use. But for DMR and hunting builds? It's the best option that fits an AR-15 lower.”

Build Notes

18"+ barrel is mandatory. Use type II 6.5 Grendel bolt (type I is obsolete). Stick with proven magazines (C Products Defense, E-Lander). Pairs well with 1-8x or higher magnification optics.

Optimal barrel: 18" - 24"

7.62x39

The AK round in AR clothing. Cheap ammo, proven terminal effects, but reliability requires attention to magazines and bolt life.

Budget intermediate / AK ballisticsSBR: good
Velocity (16")
2,350 fps
Energy (16")
1,520 ft-lbs
Effective Range
350 yds
Cost per Round
$0.35
Availability8/10
Suppressor Friendly6/10
Recoil Management6/10

Pros

  • • Cheap steel-case ammo for high-volume training (when import bans aren't active).
  • • Proven terminal performance—the round has been dropping bad guys since 1947.
  • • Good short-barrel performance compared to 5.56.
  • • .30 caliber bullet punches through intermediate barriers better than 5.56.
  • • Fits AR-15 lower with dedicated upper.

Cons

  • • Magazine reliability is the Achilles heel—curved mags in a straight magwell are problematic.
  • • Bolt breakage is more common than with 5.56 due to case taper stresses.
  • • Accuracy typically 2-3 MOA; not a precision cartridge in AR platform.
  • • Corrosive surplus ammo requires immediate cleaning.
  • • Import bans can crater ammo availability overnight.

Best For

  • ✓ Budget-focused high-volume shooting
  • ✓ Dedicated range toy
  • ✓ Brush gun / hunting (with quality ammo)
  • ✓ Shooters who reload and want .30 cal versatility

Avoid For

  • ✗ Duty/defensive use (reliability concerns)
  • ✗ Precision shooting
  • ✗ First AR build
  • ✗ Anyone who doesn't want to troubleshoot magazine issues

Hot Take

“If you want 7.62x39, buy an AK. The AR-15 platform fights this cartridge at every turn—magazines suck, bolts break, and accuracy suffers. The only reason to run it is cheap steel-case ammo, and that advantage disappears with every import ban.”

Build Notes

Use quality magazines (C Products Defense, ASC, Duramag). Expect to replace bolts more frequently than 5.56. Enhanced firing pins help with hard Soviet primers. Consider an AK instead.

Optimal barrel: 12.5" - 16"

6mm ARC

Hornady's modern answer to long-range AR performance. Better ballistics than Grendel with less recoil, adopted by SOCOM for DMR use.

Modern precision / military DMRSBR: marginal
Velocity (16")
2,750 fps
Energy (16")
1,650 ft-lbs
Effective Range
700 yds
Cost per Round
$1.25
Availability4/10
Suppressor Friendly7/10
Recoil Management8/10

Pros

  • • Superior ballistic coefficient to 6.5 Grendel with higher velocity.
  • • Less recoil than Grendel while matching or exceeding downrange energy.
  • • SOCOM adoption means military investment in the cartridge's future.
  • • Uses Grendel bolt face—shares some parts commonality.
  • • Flatter shooting than any other AR-15 compatible cartridge.

Cons

  • • Very limited ammunition availability—Hornady is essentially the only source.
  • • Expensive ammo with no budget training options.
  • • New cartridge with limited aftermarket compared to established options.
  • • Barrel life is shorter than 5.56 or Grendel due to higher pressures.
  • • Overkill for carbine-distance engagements.

Best For

  • ✓ Precision rifle competition
  • ✓ Designated marksman builds
  • ✓ Long-range hunting
  • ✓ Shooters who want the newest/best

Avoid For

  • ✗ General purpose use
  • ✗ Budget builds
  • ✗ High-volume training
  • ✗ Anyone who needs ammunition availability

Hot Take

“The 6mm ARC is what the Grendel should have been from the start. But it's a victim of timing—arriving when ammo supply chains are already strained. If Hornady can keep up production and others get licensed, this becomes the AR-15 DMR standard. Until then, it's for early adopters with deep pockets.”

Build Notes

Shares bolt with 6.5 Grendel (verify manufacturer). 16" minimum, 18"+ preferred. Requires dedicated magazines. Wait for ammo availability to improve before committing to this platform.

Optimal barrel: 16" - 20"

⚠ Critical safety: .300 BLK and 5.56 magazine mix-ups

Warning: 300 Blackout ammo in a 5.56 chamber causes catastrophic failure

A .300 Blackout round will chamber in a 5.56 barrel. When fired, the larger .308 bullet cannot exit the .224 bore. The result is catastrophic failure—destroyed rifle, potential injury or death.

This happens because both cartridges use the same parent case (.223 Rem). The .300 BLK is simply necked up to accept larger bullets. Standard AR-15 magazines feed both calibers without modification.

Prevention: Color-code all .300 BLK magazines and uppers. Use distinct mag colors (FDE for .300, black for 5.56). Engrave or paint uppers. Never store mixed ammunition. Consider dedicated range bags per caliber.

Mission-specific recommendations

Home Defense

Indoor / close-quarters defensive use

5.56 NATOAlt: .300 BLK

5.56 fragments reliably at indoor distances, reducing over-penetration risk. .300 BLK is the upgrade if you're running suppressed and want hearing-safe subsonic capability.

General Purpose Carbine

Do-everything rifle for mixed use cases

5.56 NATOAlt: 6.5 Grendel

Nothing beats 5.56 for general purpose—cheap ammo, universal parts, proven performance. Grendel if you need more range and are willing to pay the parts/ammo premium.

Suppressed SBR

Short barrel with suppressor, maximum stealth

.300 BLKAlt: 5.56 NATO

.300 BLK was designed for exactly this mission. Subsonic performance is unmatched. 5.56 works suppressed but remains loud and loses significant velocity in short barrels.

Precision / DMR

Designated marksman or precision rifle role

6mm ARCAlt: 6.5 Grendel

6mm ARC has the best ballistics for precision work if you can source ammo. 6.5 Grendel is the proven fallback with better ammunition availability.

Hunting

Ethical game harvesting at typical hunting distances

6.5 GrendelAlt: .300 BLK

6.5 Grendel offers ethical killing power on deer-sized game to 400+ yards. .300 BLK is excellent for hogs and whitetail inside 150 yards, especially suppressed.

Budget Training

High-volume practice without breaking the bank

5.56 NATOAlt: 7.62x39

5.56 steel-case runs around $0.30/round. 7.62x39 can be cheaper when imports flow, but magazine reliability issues make it less enjoyable for high-volume days.

Competition

Practical rifle matches and timed shooting

5.56 NATOAlt: 6mm ARC

Most practical rifle matches are built around 5.56 distances and power factors. 6mm ARC shines in PRS-style precision matches with longer range requirements.

How to decide

Start with mission

What are you building this rifle for? Home defense, hunting, competition, truck gun, suppressor host? Your answer eliminates most options immediately.

Consider logistics

Can you afford to train with this caliber? Is ammo available locally? Do you already own a suppressor or plan to buy one? Logistics matter more than ballistics for most shooters.

Default to 5.56

If you can't articulate a specific reason to go elsewhere, build in 5.56. It does everything adequately and nothing terribly. Other calibers are specialists—useful only if you have a specific need.

Ready to build? Open the guided assistant to configure a platform and match components to your chosen caliber.

Recommended complete uppers by caliber

Quality uppers for each caliber covered in this guide. Complete uppers simplify builds and guarantee headspace—just attach to your lower and go.

5.56 NATO · General Purpose

BCM MCMR 14.5" Upper (5.56)

BCM's MCMR upper is the gold standard for duty-grade 5.56. Mid-length gas, cold hammer forged barrel, and a handguard with integrated QD points. Pin-and-weld ready with the right muzzle device.

  • ✓Chrome-lined CHF barrel for longevity
  • ✓M-LOK handguard with anti-rotation tabs
Gas System
Mid-length · .750 gas block
Sweet spot
Pin-and-weld duty builds, patrol carbines
Verified Retailer
Check Price at OpticsPlanet
.300 Blackout · Suppressed SBR

Aero Precision 8" .300 BLK Upper

Purpose-built for suppressor use. The 8" barrel extracts maximum velocity from .300 BLK without the length penalty. Pistol-length gas cycles subs and supers reliably.

  • ✓Optimized for subsonic and supersonic loads
  • ✓Compatible with standard 5.56 BCG and mags
Gas System
Pistol-length · adjustable block recommended
Sweet spot
Dedicated suppressor hosts, PDW builds
Verified Retailer
Check Price at OpticsPlanet
6.5 Grendel · Precision / Hunting

Alexander Arms 18" 6.5 Grendel Upper

Alexander Arms invented the Grendel and still makes some of the best uppers for it. The 18" barrel maximizes velocity while keeping weight manageable for field use.

  • ✓Type II bolt included—no compatibility guessing
  • ✓Designed for 123gr hunting and match loads
Gas System
Rifle-length for smooth cycling
Sweet spot
DMR builds, deer hunting, PRS-style competition
Verified Retailer
Check Price at OpticsPlanet

Purchasing through these links may generate a commission at no extra cost to you. Always verify local compliance before ordering.

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Ready to continue? Here's the recommended next guide:

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