338 ARC vs 6.5 Grendel compares two AR15 cartridges that share the same .441-inch case head and the same Type II bolt face but do very different jobs. The 6.5 Grendel fires slender .264-inch bullets at supersonic speed for reach and wind resistance. The 338 ARC fires heavy .338-inch bullets, led by a 307 gr subsonic load, for suppressed shooting at moderate range. Chattahoochee Munitions manufactures complete firearms in both, on the CM15 AR15 and the CM7S bolt action, and uses the Grendel Type II bolt specification across both lineups.
338 ARC vs 6.5 Grendel at a Glance
| Feature | 338 ARC | 6.5 Grendel |
|---|---|---|
| Bullet diameter | .338 inch | .264 inch |
| Case head | .441 inch | .441 inch |
| Bolt face | 6.5 Grendel Type II | 6.5 Grendel Type II |
| Maximum overall length | 2.260 inch | 2.260 inch |
| Primary role | Heavy subsonic, suppressed | Supersonic, medium range |
| Typical bullets | 307 gr subsonic; 170 to 215 gr supersonic | Long, high-BC 6.5mm bullets |
| Chattahoochee Munitions CM15 barrels | 8.5, 10.5, 16 inch | 16, 18, 20 inch |
| Chattahoochee Munitions CM7S barrels | 8, 12.5, 16 inch | 12.5 inch |
| Chattahoochee Munitions twist | 1:5, 1:6, 1:8 | 1:8 (CM15), 1:7 (CM7S) |
| SAAMI | Published January 2025 | Standardized |
Same Case Head, Different Purpose
Hornady’s ARC family uses the same .441-inch case head as the 6.5 Grendel, which is why both cartridges run on the 6.5 Grendel Type II bolt face. What changes is the neck. The 6.5 Grendel holds a .264-inch bullet and is designed for supersonic velocity and long, aerodynamic bullets. The 338 ARC opens the neck to .338 inch so the same AR15 envelope can carry a much heavier bullet below the speed of sound.
Subsonic Performance
This is where 338 ARC vs 6.5 Grendel separates clearly. The 307 gr Sub-X carries 752 ft-lb at 1,050 fps, per Hornady. A subsonic 6.5mm bullet is much lighter, so it carries a fraction of that energy below the speed of sound. For suppressed subsonic shooting, the 338 ARC is the cartridge made for the job.
Supersonic Performance and Range
For reach, 338 ARC vs 6.5 Grendel favors the Grendel. Its long, high ballistic coefficient bullets hold velocity and resist wind at distances where the 338 ARC’s heavier, blunter bullets drop quickly. Chattahoochee Munitions manufactures its CM15 6.5 Grendel rifles in 16, 18 and 20-inch barrels for that reason. The 338 ARC’s 175 gr supersonic load, rated at 2,075 fps from 16 inches, suits moderate range rather than long range.
Barrel Length
In 338 ARC vs 6.5 Grendel barrel choice, the two cartridges point opposite ways. Supersonic 6.5 Grendel loads gain from longer barrels. Subsonic 338 ARC loads are capped near the speed of sound, and in Chattahoochee Munitions testing 338 ARC powder use is largely complete around 12 inches, which is why 338 ARC goes down to 8 inches on the CM7S and 8.5 inches on the CM15. See 338 ARC barrel length and the 6.5 Grendel barrel length comparison.
Shared Parts and Conversion
The shared parts are the practical side of 338 ARC vs 6.5 Grendel. Because both use the Type II bolt face, a 6.5 Grendel AR15 and a 338 ARC AR15 share the bolt face specification and the lower receiver. The barrel and the magazine change. Use a magazine made for the cartridge: the CM15 338 ARC ships with a 10-round 338 ARC AR15 magazine. Have headspace checked whenever a bolt moves between barrels. Chattahoochee Munitions sells complete 338 ARC uppers in 8.5, 10.5 and 16 inches, such as the 10.5-inch 338 ARC upper, with an optional bolt carrier group.
Bolt Action Versions
Both cartridges run on the CM7S bolt action, with Mos-Tek 416R stainless barrels, a TriggerTech R700 Primary trigger and AICS-pattern magazines. The 338 ARC CM7S uses a 1:6 twist at 12.5 and 16 inches and 1:5 at 8 inches; the 6.5 Grendel CM7S uses a 1:7 twist at 12.5 inches.
338 ARC vs 6.5 Grendel: Which Should You Choose?
- Choose 338 ARC for suppressed subsonic shooting, heavy bullets for hogs and deer at moderate range, and short barrels.
- Choose 6.5 Grendel for supersonic reach, flatter trajectory, wind resistance and longer barrels.
Explore 338 ARC in Containment Zone 338, including the CM15 338 ARC 10.5-inch AR15 and the CM7S 338 ARC 12.5-inch bolt action. Explore 6.5 Grendel in Grendel’s Lair and read the 6.5 Grendel guide. For more 338 ARC detail, read 338 ARC subsonic and 338 ARC vs 300 Blackout.
338 ARC vs 6.5 Grendel FAQ
Is 338 ARC based on 6.5 Grendel?
338 ARC belongs to Hornady’s ARC family, which shares the 6.5 Grendel’s .441-inch case head. Both use the 6.5 Grendel Type II bolt face.
Can I use a 6.5 Grendel bolt for 338 ARC?
Both use the Type II bolt face specification. Have headspace checked with the 338 ARC barrel before firing.
Do 338 ARC and 6.5 Grendel use the same magazine?
Use a magazine made for each cartridge. The CM15 338 ARC ships with a 10-round 338 ARC AR15 magazine.
Which hits harder, 338 ARC or 6.5 Grendel?
Subsonic, the 338 ARC by a wide margin, with a 307 gr bullet at 752 ft-lb. At long range, the 6.5 Grendel’s high-BC bullets retain velocity better.
Which is better for hunting?
338 ARC for suppressed close-range hunting of hogs and deer; 6.5 Grendel for longer shots on deer-size game. See 338 ARC hunting.
Which is better for long range?
6.5 Grendel. Its slender 6.5mm bullets shoot flatter and drift less in wind.
Who makes 338 ARC and 6.5 Grendel rifles?
Chattahoochee Munitions manufactures both: CM15 AR15s and CM7S bolt actions in 338 ARC and in 6.5 Grendel, with the Grendel Type II bolt specification across both lineups.