9mm Velocity by Barrel Length
Velocity and barrel length data for 9x19mm Parabellum, also known as 9mm Luger and so on. The following data is sourced from Ballistics by the Inch. To actually answer the question of velocity by barrel length for 9mm Luger, and what the ideal barrel length is, namely for something like an AR-15 / AR-9 pistol or SBR, we'll discuss below.
|Barrel Length||Cor-Bon 90 gr JHP +P||Cor-Bon 115 gr JHP +P||Cor-Bon 125 gr JHP +P||Cor-Bon 115 gr DPX||Federal 115 gr JHP||Federal 135 gr Hydra-Shok JHP (low recoil)||Federal 105 gr Expand'g FMJ||Federal 124 gr Hydra-Shok JHP||Federal 147 gr Hydra-Shok JHP||Speer 124 gr Gold Dot Short Barrel|
Best Barrel Length For 9mm
Based on the data above, it appears that almost all loads tested got their greatest percentage increase in velocity up until about a 7" barrel length. Velocity did of course increase further with longer barrels, but to a much smaller degree. For example, going from a 4" barrel to 7" (a 3 inch difference), every load got well over a 100 fps increase in velocity. However, going a full 10 inches longer up to a 17" barrel gained most of the tested loads under 100 feet per second in velocity. In fact, going from a 17 inch barrel up to 18 inches, actually LOST velocity in all but two of the loads, although the decrease was negligible in those instances.
It appears that overall, the optimal barrel length for 9mm (for something like an AR-15 pistol build, SBR, or similar setup), is about 7 to 8 inches for maximum efficiency and handling, though going longer can still have benefits. If you still wanted to maintain an ultra-short setup with a suppressor, you could go even shorter with barrel length, as the additional length of the suppressor will increase velocity. Even at 7-8" barrel, the additional length of a suppressor would still allow the pistol or SBR to remain more compact than a 16" barrel. A 7" barrel with a 7" suppressor would still give an overall length of 14 inches, so going up to 10" for a little extra velocity would put you at just about 17" with a suppressor, which is still going to handle better than a 16" barrel, along with the benefits of being suppressed. If you were looking for a typical rifle setup, there appears to be no major reason to go above the federal 16" barrel requirement, unless local laws differ, or whatever the case may be.