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!
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.
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.
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 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!