Ammo Oracle

Introduction

Assuming true M193 or M855 ammo, velocity is the key. Velocity is dependent on barrel length and environmental conditions.

As barrel length increases, the bullet is propelled faster by the expanding gasses in the barrel, imparting more velocity on the bullet, resulting in a longer range before a fired bullet drops below 2700 fps. A shorter barrel imparts less velocity, and therefore the bullet has less range.

Temperature, altitude and humidity are other factors. As temperature or altitude increases, air becomes less dense and bullets travel faster. Contrary to common conceptions, as humidity increases air also becomes less dense and helps bullets retain velocity.

It is important, then, to keep in mind that any statistics given can only be approximate and can be affected by a wide range of factors. But as a baseline, these numbers are what you could expect for 75° F, 25% humidity, at sea level, from various barrel lengths:

As you can see, barrel length and ammo selection make a major impact on fragmentation range.

Opinions (Pro and Con):

14.5" and 11.5" barrels are great, why waste all that weight and effort lugging around something larger?:
Shorter barrels are critical to close quarters battle (CQB) and urban work. Here ranges are shorter and getting that barrel around corners and inside houses is tough enough without it being too long. All that velocity isn't necessary and I'm worried about overpenetration anyhow so it's a good thing to keep it under control. If I really am worried about low velocities I'll just switch to controlled expansion rounds.

I wouldn't be seen with anything shorter than a 16" and I'd try to hide my face if friends saw me without a 20":
I need a more flexible weapon that allows me to get out to 200 meters when I have to. Additionally, fragmentation is critical to my philosophy about wound ballistics and I want as much of it as I can get. This means at least 16" on my barrels and 20" is better. 16" is plenty short enough for interior and urban work and it gives me the advantage of not having to worry about using my rifle for long shots on deer or for longer defensive engagements. Plus, if I had a true 14.5" barrel I'd just have to register it with the ATF as a short barreled weapon anyhow. Why should I have 16" of overall length and only get the benefit of 14.5" of that?