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How to Build a 6.5 Creedmoor AR-15

How to Build a 6.5 Creedmoor AR-15

Posted by on Jun 19th 2024

The field of 6mm cartridges has grown to become some of the most popular long-range rifle calibers today. The 6.5 Creedmoor is one of the most popular of them all -- and for good reason: It yields high ballistic coefficients, providing accuracy up to 1,000 yards. It offer plenty of take-down power, making it a great hunting round. It's roughly the same size as the .308 Winchester, while offering less recoil and more performance.

Building a 6.5 Creedmoor AR

That last point is why so many shooters want to build a 6.5 Creedmoor AR. It's a great alternative to the typical AR-10 chambered in .308, while using nearly all the same components.

Picking The Right Creedmoor Barrel

Shop barreled 6.5 Creedmoor uppers

The barrel's the most important component of any Creedmoor build. When it comes building an AR, barrel length is always a compromise between size and weight, and performance. To find the ideal barrel, let's look at the muzzle velocities of 6.5 Creedmoor when fired from different lengths:

Barrel Length vs. Muzzle Velocity (147-grain ELD Match):

  • 26" - 2,728 ft/s
  • 25" - 2,691 ft/s
  • 24" - 2,677 ft/s
  • 23" - 2,637 ft/s
  • 22" - 2,638 ft/s
  • 21" - 2,609 ft/s
  • 20" - 2,577 ft/s
  • 19" - 2,542 ft/s
  • 18" - 2,502 ft/s
  • 17" - 2,492 ft/s

Choosing a Barrel Length

The data above shows that when shortening the barrel from 26" to just 17", the 6.5 Creedmoor only loses ~8.5% of its muzzle velocity. When shortening to a 20" barrel, the round loses about 5.5% of its muzzle velocity.

In other words, you can pick a relatively short barrel and still enjoy high velocity and, thus, good accuracy at long distances.

The general consensus is that Creedmoor performs best with a 24" to 26" barrel. But these barrel lengths are not commonly found on AR-type rifles. We recommend a 20" to 22" barrel for your Creedmoor AR.

The Ideal Twist Rate

Plenty of data and field testing has shown that Creedmoor rounds provide the best accuracy and in-flight stability with a 1:8 twist rate. This twist rate works best for barrels in the 20" to 22" range, given the slight reduction in muzzle velocity. Long barrels (24" or more) may benefit from a 1:8.5  or 1:9 twist rate.

Which Gas System is Best?

When running a 20"+ barrel, the Creedmoor round requires a rifle-length gas system. If you opt for a shorter barrel -- 17" to 18" -- you'll need to run a mid-length gas system.

Picking an Upper and Bolt Carrier Group

The 6.5 Creedmoor utilizes the same stripped upper receiver and bolt carrier group as the .308 Winchester cartridge.

(A "High-Pressure" Bolt is Not Necessary)

Some shooters claim a high-pressure bolt is required to fire and cycle 6.5 Creedmoor with a .308 bolt carrier group. This is not necessary. Both rounds produce a SAAMI chamber pressure of 62,000 PSI, so there is no additional pressure produced by the Creedmoor's cartridge at the bolt face.

Choosing a Lower Receiver Assembly

The 6.5 Creedmoor also uses the same stripped lower receiver, lower parts kit, and buffer assembly as the .308 AR.

The Creedmoor Uses .308 Magazines

Because the Creedmoor and .308 cartridges are so similar in size and shape, there's no need to invest in custom magazines. Standard AR-10 magazines work with the cartridge, with 5- and 10-round capacities providing the best reliability when it comes to feeding.

Picking The Right 6.5mm Buffer

The 6.5 Creedmoor cartridge works well with an "H" buffer. If you're upgrading an existing .308 AR to chamber Creedmoor, your rifle is already likely to have an "H" buffer installed.

Building a Creedmoor Upper?

Putting together a new Creedmoor upper for your existing .308 lower? Read our how-to guide on installing your new barrel and gas system! This guide covers everything you need to assemble your new upper.

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

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