A man looking through the scope of an AR rifle

Choosing a Caliber for Your Large-Frame AR Build

6.5 Creedmoor & 308 Win are the most popular cartridges for your M5 build.

The jump from a standard AR15 to a large-frame M5 gives you the ability to run more full-size rifle cartridges. Most commonly you will find that 6.5 Creedmoor and 308 Win are the two most accessible cartridges for your build. So, which will it be, the time-tested 308, or the newer 6.5 Creedmoor that has taken the world by storm? In this article, we will be exploring some very surface-level characteristics of both cartridges that should help you decide which caliber for your first M5 build!

The Cartridges

From the neck down, 308 and 6.5 Creedmoor are very similar. Both cartridges use the same ~.473” rim diameter, and have roughly the same case capacity. The case length of the 308 is a smidge longer, but the average cartridge overall length of 6.5 Creedmoor is greater.

Without getting into the weeds on case geometry- the primary differentiator between the two is the projectile diameter. The 308 Win uses a .308” projectile, whereas the 6.5 Creedmoor uses a .264” projectile.

Both cartridges can reliably feed from SR25 pattern magazines making them great candidates for M5 builds.

Ballistic Performance

Utility is generally the first consideration- and in this case, there isn’t a clear winner. Speaking very generally, 308 is capable of firing heavier bullets- whereas 6.5 has a flatter trajectory. Can both fill a lot of the same roles? Absolutely.

If you want to focus on long-range precision- the flatter trajectory and higher ballistic coefficient (how resistant to drag the projectile is) of the 6.5 Creedmoor is hard to beat. While you absolutely can make a lot of the same shots as a 6.5 with a 308, the 6.5 will do it much more efficiently. What this translates to is smaller holdover and windage adjustments when shooting at distance.

Predominately a consideration for hunters-308 does allow you to run a heavier projectile, but it does go slower. This often lends to higher muzzle energy depending on the load you are using- however at extended distances 6.5 may maintain its velocity (and transitively energy) better.


An often overlooked aspect of this discussion is recoil- 6.5 Creedmoor is the clear winner in this department. On a gas gun specifically, this makes the 6.5 shine as it allows for quicker reacquisition of your target. While the 308 isn’t exactly staggering- there is a noticeable difference if you shoot the same platform in both cartridges side by side.

A man long range shooting with large frame AR

Barrel length

308 lends itself better out of shorter (sub 20”) barrels than 6.5 does. The reason for this is that 308 can reach practical velocities in a shorter barrel length than 6.5. If you are explicitly wanting to build something relatively short and compact (16-18”) 308 might just be the ticket. 6.5 can still work out of short barrels- it will just do so much less efficiently, generally speaking, 20” is a low-end barrel length for 6.5 Creedmoor.

Barrel Life

Barrel life is highly dependent on firing cadence, ammunition, and maintenance. For the sake of discussion, if you took 2 M5s (one 6.5 Creedmoor, and one 308)  in identical configurations other than caliber and shot them at the exact same firing cadence with comparable ammunition the 308s barrel will last longer. This is because both cartridges have similar case capacity, but the 6.5 has a notably smaller bore diameter which translates to less surface area and more wear on the internal surfaces of the barrel. To put it simply, you are burning roughly the same powder charge in a much smaller area with the 6.5 Creedmoor.

With this all being said, over a long enough time you will notice shorter barrel life with 6.5 Creedmoor, but we are still talking thousands of rounds before noticeable degradation. For a hunting rifle, even 1000 rounds is a lifetime worth of shooting in most cases. For precision shooting you should always look at your barrel as a consumable item- and if you can afford to shoot thousands of rounds of match-grade ammo through your rifle, a rebarrel is a small expense comparatively.

Ammunition Cost and Availability

The cost of ammunition is always a hot-button topic with the 308 versus 6.5 discussions. At the end of the day, if you compare the cheapest ammunition for both cartridges, 308 is cheaper. However, match ammunition for both cartridges is pretty comparable in price. If you plan to use your build for precision shooting- the ammunition cost difference is nearly a non-issue.

As far as availability is concerned there is more 308Win/7.6251NATO in global circulation as it is a widely adopted military cartridge. However, domestically in the United States, you can easily find 6.5 Creedmoor at most local and big box gun stores. You will likely be able to find more ball 308 ammunition floating around, but if you plan on shooting match-grade ammunition this is once again nearly a non-issue.


The decision ultimately comes down to what you- the builder wants. Rifles should almost always be purpose driven when being built- so take the above considerations and the intended use-case to decide which cartridge is best for you!

What caliber for AR rifle build

Comments (20):

  1. Sean P

    June 26, 2023 at 12:19 pm

    I’m curious to know if Aero (or Ballistic Advantage) is looking into adding any 8.6 Blackout. I think there’s a lot of potential there.

  2. Richard Kayser

    June 26, 2023 at 12:39 pm

    Two stellar choices to be sure. I had a lot of experience in Vietnam with the M14 select-fire. As far as the creedmoor, it does not offer much or any advantage over the 260 Remington I built fifteen years ago, since I reload. My latest choice, built on your platform, is a 358 Winchester. Like the 260, it uses standard Magpul 308 magazines. There are no flies with a 250gr spitzer bullet at 2250fps.

  3. Richard Kayser

    June 26, 2023 at 12:40 pm

    Airborne!! Rangers Lead the Way…

  4. Blake

    July 2, 2023 at 5:43 pm

    Any way you could do some true budget stuff for us broke guys that want to support you still lol.

  5. McGuire

    July 5, 2023 at 10:45 am

    What is the best length for a 6.5 creedmoor for long range shooting

    • Randy Griffin

      July 20, 2023 at 3:03 pm

      The longer the barrel the better for 6.5 CR. I have 2 of them and the barrels are only 20 inches. A 26 inch barrel will send it down range much faster and out to 1000 yards, the 308 drops 6 feet more than the 6.5 CR. Our military is putting this caliber to work as sniper rifles and ai heard they love them.

  6. William Patrick Brabston

    July 7, 2023 at 10:20 am

    Great article.
    Practical, easy to understand & thoroughly explains the subject.

  7. Jeff Fowler

    July 8, 2023 at 8:29 am

    I am surprised you didn’t mention 7mm-08 at all. That would give an example of the versatility of the platform.

    • Eric

      September 23, 2023 at 4:00 am

      Exactly… that’s the caliber I am about to build out my AR10 for.

  8. Rodney Stevenson

    July 11, 2023 at 12:11 pm

    I am far more interested in building a 308

  9. William

    July 11, 2023 at 2:36 pm

    Great information.. thank you fir taking the time.. its useful..

  10. BILL

    July 11, 2023 at 5:49 pm


  11. Adam

    July 11, 2023 at 8:48 pm

    I was building a .308 AR but now, after reading this article, I think I’m going to go with the 6.5. I hadn’t really looked into it as an option before. I’m kinda old school and like sticking with what I know. That being said, new and different is only new and different once. Great article, thank you.

  12. George Brudnak

    July 12, 2023 at 2:14 pm

    I will be staying with 308,has worked fine for me for 48 years.

  13. Bears Corner

    July 20, 2023 at 8:52 am

    Started out with 303B & looked to get a Globe Mohawk 555/ Tokarev. Traded them in for AR45ACP+P & AR22LR & am now interested in ARPistol versions for them with compatible magazines. ~Glock21, 41 & RIA TM22 w/ 8″ TBarrel 1:16.

  14. Jeffrey

    July 30, 2023 at 10:22 am

    Great article, i am now even more undecided lol, will the 308 fire the 7.62 using the same magazine? For that Matter is the 308 and the 7.62 alike, are they like the .223 and 5.56 if so should the barrel I get say 7.62 on it or is the 308 good for any of the amo.

    • elronmack

      August 27, 2023 at 8:26 pm

      I always heard to get the rifles chambered in 5.56 and .308, respectively.

  15. Mike C.

    August 9, 2023 at 1:10 am

    Love my M5 .308. Where can I find a list of recommended ammo specs to shoot with. I shot some “Ammo” brand bonded blue line stuff and it would jam every time. My buddy pulled my P-mag off my rifle and loaded a steel mag with M80 ball and it cycled like a dream.

  16. Mike C.

    September 20, 2023 at 8:44 am

    I redact my last comment. It’s still double feeding on Remington M80 and PMC match grade X-Tac with P-mags and Steel mags. I tried to contact Aero technical support over the phone with no luck. I’m bringing it to my gun shop in hopes they can figure it out.

  17. Mark J Kinchen

    November 1, 2023 at 10:05 pm

    I already built an Aero AR 15, should my next one be an AR 10… both for ‘defense’.


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