The Best Barrel Length for The AR-9 (Ballistic Data)

When it comes to building a pistol-caliber carbine using the AR platform, 9mm is the best cartridge. It's a high-capacity round that needs no gas system and produces almost no felt recoil. It can be suppressed with subsonics, or it can maximize 9mm's velocity to push the limits of what your AR9 pistol (or rifle) is capable of.

Achieving optimal velocities means picking the right barrel length. What is the optimal length? Let's review the ballistic data.

9mm Barrel Length vs. Velocity Chart

This chart illustrates 9mm's barrel lengths and muzzle velocities across the most popular bullet grain weights, which range from light 90-grain loads, up to heavy 147-grain loads.

If we compare all muzzle velocities for all common 9mm loads, we find some key data that allow us to find three optimal barrel lengths for 9mm, depending on application.

4" is Best for Subsonic Loads And Suppressors

For common and popular 9mm loads, a 4" barrel reliably produces muzzle velocities below the transonic sound barrier, which is about 1,125 feet per second (depending on temperature and humdity). At this velocity and beyond it, rounds produce supersonic muzzle velocities and significantly increase their sound signatures, reducing suppressor performance.

We stress the "reliably" part of this recommendation, because the speed of sound can be as slow as 1,086 FPS in cold, dry air. The 4" barrel keeps most loads below this critical speed. Note that even with the shortest practical barrel length, some loads, like 90-and 115-grain JHP +Ps, simply cannot be made to exit the muzzle at subsonic velocities.

9" is The Best Overall Barrel Length

We can see that for many of the most popular 9mm loads -- including 147-grain Hyra-Shock and 124-grain Speer Gold Dot -- achieve optimal muzzle velocities with a 9" barrel. And we can even see that if we increase barrel length to 10", we actually lose muzzle velocity. Although going yet longer can again produce some extra velocity, it isn't a meaningful nor useful amount.

12" Maximizes 9mm's Performance

With 12" of barrel, all 9mm loads will achieve their maximum muzzle velocity and produce the most accuracy and capability possible with this cartridge. Beyond this barrel length, only marginal gains in velocity are possible.

(A 16" Barrel is Still Perfectly OK)

Don't want to build an AR9 pistol? Want to use a buttstock, but don't want to go through the hassle of registering your 9mm AR as a short-barreled rifle? For these two reasons, it's still perfectly acceptable, and even logical and convenient, to simply go for a 16" barrel when building your AR9.

The 9mm AR rifle has become more popular for its extra stability and even lower felt reocil, especially as increasingly accurate and more powerful 9mm loads hit the market -- and it is true that you can still squeeze out a few extra feet per seconds' worth of velocity with some 9mm loads shown above (like 115-grain DPX, or any +P loads).