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300 Blackout vs 9mm Parabellum

The pistol-cartridge segment of the AR market is picking up steam – we even sell a very cool and proprietary AR-9 Complete Build Kit on our sister site, We know that the MP5 reigned supreme as a close-quarters, suppressed weapon and PDW for decades. But now, 300 Blackout SBRs and pistols seem to be filling that slot for the military. So, which is better?

300 Blackout and 9mm by The Numbers

First, let’s break down the ballistic data of both cartridges. Keep in mind, these averages are based on each round being fired from a 16” barrel. Because 9mm is usually fired from pistol barrels, most data available on the round’s energy and velocity will not be applicable. We did the calculations ourselves, based on the AR-15 platform:

300 AAC Blackout:

  • Weight (average): Supersonic, 125 grains; Subsonic, 220 grains
  • Velocity (average): Supersonic, 2,215 ft/s.; Subsonic, 1,010 ft/s.
  • Energy (average): Supersonic, 1,350 lb.-ft.; Subsonic, 500 lb.-ft.

9mm Parabellum:

  • Weight (average): Supersonic, 115 grains; Subsonic, 147 grains
  • Velocity (average): Supersonic, 1,525 ft/s.; Subsonic, 1,073 ft/s.
  • Energy (Average): Supersonic, 594 lb.-ft.; Subsonic, 376 lb.-ft.


The Ballistic Takeaway

9mm is unique for handguns, just as 300 Blackout is unique for rifles, in the same fashions: Both are available as supersonic and subsonic loads, and both can be fired from their respective platforms without modifying the subject weapon.

If we look at sub vs. sub, 300 Blackout heavier and slower, hitting with slightly more pound-feet of forge. Super vs. super, we find 300 Blackout is still just a tad heavier, much faster, and way more powerful. This is not surprising because 9mm is a pistol cartridge by design.

For this comparison, then, we have to focus on subsonic. On paper, 300 BLK bests 9mm in every way: It’s ballistically superior, and it allows the shooter to transition to a supersonic load with the switch of a magazine.

What, then, is the benefit of the 9mm AR?

Why the AR-9 is Still Cool

Realistically, most of us will not ever use either platform and test it to its limits. Most ranges available to the average Joe or Josephine max out at 100 yards. Both 9mm and subsonic 300 BLK will perform the same in these conditions.

You might also want the AR-9 because of your shooting lifestyle and current weapons: 9mm is a hugely popular pistol round, if not the most popular pistol round in existence. You might own one, two, or three 9mm handguns. It only makes sense, then, to own the AR-9.

But what if you mostly own AR 15s? Then the 300 Blackout makes sense, because the parts are mostly interchangeable – the only difference between a .223 or 5.56 AR and a 300 BLK AR is the barrel and chamber. Everything else is the exact same.

So, Which One’s Best?

Neither, and both. The 9mm and 300 Blackout AR will serve the same purpose in close quarters, and if you’re comparing these two rounds in the AR platform, then you’re obviously building a rifle intended to fulfill just that purpose: A close-quarters rifle, pistol, or general PDW.

300 Blackout Pistol Upper (7.5" Barrel & 7" Free Floating Quad Rail) AR 15 Complete Pistol Upper

Our 7.5″ 300 Blackout Pistol Upper provides the PDW profile and performance the 9mm provided in the classic MP5, in the modernized AR platform

If you want a dual-purpose weapon that can act as a PDW and a conventional rifle, the 300 Blackout pistol uppers will edge out 9mm. If you want something that’s dedicated as a PDW or close-quarters weapon with more readily available (and more affordable) ammunition configured in vastly different loads (something 9mm far, far outpaces 300 BLK in), and you desire something that’s just different from every other AR out there, then the 9mm, or AR-9, platform will be a better choice.

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 300-blackoutupper, 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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