Note: This article is still being written; expect something final in a few days
VERY few questions I get at gun shows are any more irritating than "What's the effective range of this weapon?" The asker typically means, "If I buy this gun and go grab some ammo off the shelf without researching the correct type to use, what's the farthest I can drop anyone I want to, assuming I practice a lot? By the way, I expect you to know the precise MOA potential for every single upper you stock, even though the manufacturers don't supply that info and I would expect the rifle for half price if you had actually test-fired it to find this out for me."
It's worth knowing that whenever a seller actually answers this question, it's almost always a ballpark guesstimate. Effective range, as defined below, is a stack of three variables: weapon/ammo accuracy, shooter proficiency, and lethality. Muzzle velocity is important here, mainly because it determines the degree of muzzle drop, as well as the point at which the round goes subsonic. When a spitzer (i.e. conventional pointy) bullet's velocity drops below the speed of sound, its trajectory is disturbed. From that point it begins to tumble, and its accuracy goes downhill very quickly from there.
Definitions of Effective Range for Non-Sniper Rifles
When folks ask about "effective range", they typically mean the maximum effective range on a point target. It is worth knowing that sniper rifles are judged differently from pretty much all other rifles. Below, are the maximum effective range values you would use on a semi-automatic battle rifle or assault rifle... anything other than a sniper rifle.
Absolute maximum effective range: This the "this round is not considered lethal after crossing this threshold" distance. Neither of the other two common "maximum range" values will be greater than this. Purportedly, NATO defines this as the point at which the projectile's kinetic energy dips below 85 joules (62.7 foot-pounds). This is typically claimed when recounting that the P90's effective range is 400 meters on unarmored targets, as classified by NATO. It's worth noting that while the P90 looks neater than the civilian PS90, the extra barrel length increases the muzzle velocity and thus the civilian model actually has a longer absolute max effective range.
Maximum effective range on a point target: This is the maximum range at which an average shooter can hit a human-sized target 50% of the time. "Point target" is basically a euphemism for hitting a human torso sized area in this context. If this range were greater than the absolute maximum, the absolute maximum would be quoted (a non-lethal hit may be accurate, but it's not effective).
Maximum effective range on an area target: This is the maximum range at which an average shooter can hit a vehicle-sized target 50% of the time. In other words, this is the maximum distance at which it would make sense to open fire on a group or vehicle, etc. If this range were greater than the absolute maximum, the absolute maximum would be quoted (a non-lethal hit may be accurate, but it's not effective).
It is worth noting that relatively low foot-pound or joules ratings are listed as lethal. This is best thought of with the analogy of a jovial back-slap. Assume that you're giving a friendly slap on the back to a friend, with an open hand. It will be noticed, but probably won't inflict serious pain or leave a mark. However, if you used the exact same force, but were clutching an icepick (very small surface area, instead of a wide surface area) and directing it perpendicularly to your friend's back, chances are that your icepick would wind up embedded to the hilt and you'd have some serious explaining to do. The same is true for bullets - a bullet (relatively small cross-sectional area) with what sounds like a small amount of kinetic energy behind it, can deliver considerable energy per square inch when making contact. I have however found numerous discussions on the internet where 85J is cited as NATO's cutoff (most of which center around the P90 rifle), but nothing to contradict or confirm it - and hence it is listed only as a NATO's purported minimum lethal kinetic energy.
It should be apparent that "effective range" is highly dependent on the shooter and is also closely tied to the appropriate choice of ammunition for your weapon. An AR-15 with a 20" 1:12" barrel may have an effective point-target range of 800 yards when firing 55gr ammunition, but then upgrading to 69gr or 77gr ammunition would see the effective range drop dramatically due to the inadequate twist rate for the higher loads.
Sniper rifle effective range: Sniper rifles are judged by entirely different criteria. A sniper rifle's effective range is judged based upon the range at which one shot, carefully fired by an expert marksman, is guaranteed to strike the target. Sniper weapons tend not to list point or area effective ranges, as sniper rifle effectiveness is not calculated with 50/50 hit ratios.
Additional Sources on Maximum Effective Range
From the US Army M-16A2 study guide - Q: What is the definition of Maximum Effective Range? A: The greatest distance at which a soldier may be expected to deliver a target hit.
Rough Guides for Various Calibers in Sniper Rifles
It bothers me to write this, but the table from allexperts.com seems to be a good ballpark approximation for the various calibers. Bear in mind that these values are all ballpark guesstimates for weapons with approximately 1MOA accuracy, firing match grade ammunition.
Effective range is not typically considered a one-shot determination. The US military is reputed (can't find a direct cite here) to consider the range at which an average shooter has a 50% hit/miss ratio on a target to be the maximum effective range for that weapon and target size. So if the maximum effective point target range of an M-16A2 is 550 meters, then it means an average soldier can score 5/10 (or 15/30) hits on a person at that distance. Assuming a better soldier or better rifle, the effective range would increase up to but not beyond the absolute maximum effective range.
Lethal Kinetic Energy
Kinetic energy is a product of the speed of a bullet and its weight. This governs how much force is imparted into the target upon impact. This is best thought of in terms of professional baseball pitchers, who can throw balls at over 100 miles per hour. Throwing a tiny 40 grain (1gr=1/7000thlb) 223 projectile, even a professional baseball pitcher would be hard pressed to kill someone. However, when given a 750 grain 50 BMG projectile, it's possible for a 100mph impact to kill someone. The exact same concept holds true for a bullet being fired from a gun. Higher velocity and greater mass increase the kinetic energy available to ultimately deposit into the target. Now, how that energy gets distributed, is another story. According to Wikipedia, an expanded hollow point bullet traveling 250 ft/s will penetrate skin 50% of the time.
-- SeanNewton - 12 Mar 2008
NOTE TO THE GUY WHO KEEPS SENDING COMMENTS ON THIS ARTICLE AND DOESN'T SUPPLY AN EMAIL ADDRESS: I would like to share my sources with you and maybe you can find some things I couldn't. But, since you keep NOT providing a return address, I CAN'T REPLY! I'm not looking for a flame war or anything, but I would like to be able to actually talk with you.