Would barrel length affect FPS thus also affecting penetration?
The 9 mm rifle with the 16 inch barrel made it to 4 barrels. The 9 millimeter pistol only went 3 barrels. There's definitely a point of diminishing returns as you keep putting more and more power into the bullet. The AK-47 only did marginally better. The biggest factor is the air friction after the bullet starts tumbling. That slows it down..