Two answers to the same question
Both cartridges were built to make the AR-15 do something the 5.56 doesn't do well. They just aimed at different problems.
- 6.5 Grendel was designed in 2003 by Bill Alexander and partners to give the AR-15 better long-range performance. It was standardized by SAAMI in 2010. It fires sleek, high-ballistic-coefficient 6.5mm bullets.
- .300 AAC Blackout was developed by Advanced Armament Corp with Remington and approved by SAAMI in 2011. It fires .30-caliber bullets from short barrels and works well with suppressors, especially with heavy subsonic loads.
Our .300 Blackout vs 5.56 comparison covers how the Blackout stacks up against the standard AR round.
Side-by-side
| 6.5 Grendel | .300 Blackout | |
|---|---|---|
| Bullet diameter | 6.5mm (.264 in) | .30 caliber (.308 in) |
| Common bullet weights | 90–130 gr | 110–150 gr supersonic; 190–220 gr subsonic |
| Example listed velocity | 2,620 fps (Hornady 123 gr SST) | 2,100 fps (Remington Core-Lokt Copper 120 gr); 1,050 fps (Magtech 200 gr subsonic) |
| Bolt | Needs a Grendel-specific bolt (larger bolt face) | Standard AR-15 bolt |
| Magazines | Grendel-specific magazines recommended | Standard AR-15 magazines |
| Strength | Longer range, flatter, less wind drift | Short barrels, suppressors, subsonic |
| Ammo availability | Good, but smaller selection | Wide, including many subsonic loads |
6.5 Grendel: reach from an AR
The Grendel's strength is the 6.5mm bullet. Long, streamlined bullets hold their velocity well and resist wind, so the Grendel shoots flatter and drifts less than a 5.56 or .300 Blackout at longer distances. That makes it attractive for hunters who want an AR-platform deer rifle for open country, and for target shooters who want to reach farther without moving up to a larger AR-10 platform.
The catch is parts. The Grendel's case head is larger than the 5.56's, so it needs a specific bolt, and Grendel magazines are recommended for reliable feeding. Buying a complete upper or rifle built for 6.5 Grendel is the simplest route.
Ammunition Depot lists Grendel options on its 6.5 Grendel page, from FMJ practice ammo to hunting loads.
.300 Blackout: short, quiet, simple
The Blackout's biggest selling point is compatibility. It uses the same bolt and magazines as a standard AR-15; you change the barrel. It also performs well from very short barrels, and heavy subsonic loads stay below the speed of sound, which makes a suppressed .300 Blackout much quieter than a suppressed 5.56.
The trade-off is trajectory. The .300 Blackout's bullets drop more past 200 yards than the Grendel's. It's designed for close to moderate range, not long-range shooting.
See our best .300 Blackout ammo and best subsonic .300 Blackout guides, or browse Ammunition Depot's .300 Blackout page.
The .300 Blackout safety warning
This needs its own section. A .300 Blackout round can sometimes chamber in a 5.56 or .223 rifle, because the cases share dimensions at the base. Firing it in a 5.56 barrel can cause a catastrophic failure, since the .30-caliber bullet is too large for the smaller bore. SAAMI lists this combination as unsafe.
If you own both, mark magazines clearly, keep ammo in separate, labeled containers, and never mix them in a range bag. Our wrong ammo kaboom guide covers this exact failure.
The Grendel doesn't share this risk with 5.56 in the same way, because its case head is larger.
Example loads
How to choose
- You want to shoot past 300 yards from an AR: 6.5 Grendel.
- You hunt deer in open country with an AR: 6.5 Grendel, with a proper hunting bullet.
- You want a short, suppressed rifle: .300 Blackout.
- You want to reuse your AR bolt and magazines: .300 Blackout.
- You hunt hogs or deer in thick cover at short range: either works; the Blackout is handier.
Check your state's hunting rules for any caliber or cartridge restrictions before taking either into the field.
Barrel length, optics and setup
Each cartridge suggests a different kind of rifle. Matching the build to the round helps you get the most out of it.
Building around the Grendel
The Grendel shines with longer barrels, often 16 to 24 inches, where it can make full use of its powder and bullet. A magnified optic makes sense, since its advantage is at distance. Many owners treat it as a light, handy precision or hunting rifle rather than a short carbine.
Building around the Blackout
The .300 Blackout is at its best in short barrels, commonly 8 to 10 inches for suppressed builds and up to 16 inches for a standard-length rifle. A red dot or low-power variable optic suits its close-to-moderate range role. Note that barrels shorter than 16 inches on a rifle are regulated under federal law; follow all NFA rules and your state's laws before building or buying one.
Subsonic considerations
If you plan to shoot subsonic .300 Blackout, check that your rifle's gas system is set up to cycle it. Some rifles run both supersonic and subsonic loads reliably; others need an adjustable gas block or a suppressor attached to cycle subsonics. Our guide to subsonic vs supersonic ammo explains the speed-of-sound threshold.
Practice costs
Both cartridges cost more per round than 5.56. For the Grendel, buying a box of FMJ in the same weight as your hunting load is a good way to practice. For the Blackout, many owners keep a supply of supersonic FMJ for practice and a separate stock of subsonic or hunting ammo for its intended job.
Whichever you choose, buy one load in quantity once you find what your rifle shoots well, so your zero stays consistent from practice to the hunt.
Frequently asked questions
Does 6.5 Grendel need a different bolt?
Yes. Its larger case head needs a Grendel-specific bolt. Grendel magazines are also recommended for reliable feeding.
Does .300 Blackout use standard AR-15 magazines?
Yes. It uses the standard AR-15 bolt and magazines; only the barrel changes. Mark your magazines if you also own a 5.56.
Which is better for deer hunting?
The 6.5 Grendel reaches farther with a flatter trajectory. The .300 Blackout is fine at shorter ranges. Use expanding hunting bullets with either.
Can I shoot .300 Blackout in a 5.56 rifle?
Never. It can sometimes chamber, but firing it can destroy the rifle and injure you. SAAMI lists it as an unsafe combination.
Which is cheaper to shoot?
It varies by load. .300 Blackout has a broader selection, which can help with pricing. Compare cost per round on current listings.
Editorial note: This comparison draws on SAAMI information, manufacturer details, retailer listings and published references. We haven't tested these loads hands-on. Specifications, availability and prices change, so check the current listing and your rifle's manual before buying. We may earn a commission from links on this page at no extra cost to you.
Affiliate disclosure: Some links on this page are affiliate links to Ammunition Depot. We may earn a commission if you buy through them, at no extra cost to you. Our editorial choices are not influenced by commissions. Check current price, stock and specifications directly with the retailer.