The big advantage is speed, and lots of it. The 5.7mm round uses lighter weight bullets than a round of 9mm. Rather than shoot a 115 grain 9mm round at around 1100 FPS, the 5.7mm fires 40 grain rounds that scoot out of the barrel at around 2300 feet per second.

FN designed 5.7mm rounds with a ballistic profile that is more similar to a rifle bullet than a pistol bullet. A 5.7mm has a pointed tip and boat-shaped tail which help it cleave through the air, increasing its effective range 5.7 ammo.

The diameter of 5.7 vs 9mm makes a difference inside the pistol. A round of 5.7mm is less than 2/3rds the diameter of a round of 9mm. This means you can stack more of them inside a magazine. This in turn means more rounds inside the magazine of your pistol.

5.7mm is the new kid on the block. Because of this, there is not the wide variety of ammunition available for it compared to 9mm. In addition, the best ammunition for it, the SS190 armor-piercing round, is not available for sale to armed citizens. The SS190 is considered to be “armor-piercing,” and that’s a no-go for armed citizens, says the ATF. On top of this, the cost per round of 5.7 vs 9mm is higher by about 10%, which translates into less bang for your buck