There are a couple other things to remember about published effective ranges...
1. The time and context of the publication.
For example during WWII,
the manuals for the M-1 Garand were meant to be informative to a group of troops who probably had some sort of experience with firearms.
Training films from that era also show that shooting at exposed troops was not the only thing you were expected to do with a rifle. You were also expected to disable vehicles, blast through concrete walls, damage artillery and towed guns, kill troops hiding behind tree (by shooting through the trees) make tank crews button-up and engage enemy aircraft. Using the mass produced ammunition, from various sources that loaded ammo during WWII, you can / could do all of that within 450 yards with an M-1 Garand.
As an individual rifleman, a platoon of guys with M-1 Garands can keep an area 1,000 yards away under constant semi-plunging fire.
During the house top house fighting in Europe, fire teams from "The Big Red One" used their rifles to blast doorways from one house basement to the next house basement, right through concrete and stone masonry.
My Dad said that in the Pacific his unit stopped the light-weight Japanese tanks using Garands and BARs loaded with M2 Armor piercing ammo.
*Yes I know that many M-1 Garands shoot nice groups far past 450 yards with good ammo. But during WWII there were lots of ammo makers with different tolerances. Plus there was still all sorts of WWI and 1920s vintage ammo ( the M-1 ammo with the 174 grain boat-tail bullet) in the supply chain besides the M2 150 grain flat based spitzer or the M-2 armor piercing round. Besides that fact that the Army studies at the time said that 400-500 yards was the maximum range for most infantry engagements, there is also the trajectory of the M-2 ball round to consider. It was a 150 grain flat based spitzer bullet at 2,800 fps. If an M-1 Garand is zeroed at 370 yards with M-2 ball ammo, the path of the bullet will always being within 10 inches of the bore line from zero to around 440 yards, thus no sight adjustment needed for an impact somewhere on the full torso of an adult. Either 10 inches low (guts) or high (upper chest) , out to 440 yards without a sight adjustment .
During the 1950s,
the folks who dealt with the M-14 were guys like my dad who were WWII veterans. Once again the manual was made to be informative. During this time period 7.62mm ammunition was loaded to better tolerances due to the lack of war-time concerns. The M-14s were also not being made of a mass scale and the barrel tolerances were somewhat better. The same general use of the rifle was expected of the rifleman. So 500 yards was reasonable using the 147 grain ammo..... It was already being done with the M-1 Garand and good ammunition.
Sight height above the bore-line also comes into play.
Now enter the 1960s:
The original AR-15s and M-16s (what the Air Force ordered) and the later Army M-16A1's were greatly resisted by the old time (senior ) troops who had already fought in WWII, Korea, China, the Dominican Republic, Haiti, and early Viet Nam, using Garands and M-14s. There was a lot of political pressure via the McNamara controlled DoD, to push the weapon into service. Also a lot of pressure to make it seem better than it really was. Particularly since the population of troops was now much less experienced with firearms when entering the service. They needed comic books for instruction...
http://www.retronaut.co/2012/07/m16a1-rifle-comic-1968
Also the need to blast through barriers, disable vehicles, and breach walls was forgotten. Assuming that an M-60 machine-gunner would be available to do the heavy lifting.....
The original max effective range listed was 400 meters for the M-16. The M-16 original rear sight was not quick adjustable in combat. When zeroed at 25 meters the height of the sights over the bore also meant that the rifle was rough zeroed at 300 meters as well.
A shot to 400 meters with that sight setting would mean a body hit as long as you held on the upper chest of your target. At least on paper....So 400 meters was the effective listed range...
With the introduction of the M-16A2 with its 20 inch barrel , 1 in 7 twist rate and the M-855 ammo, the range was supposedly increased due to the ability of the shooter to adjust the sights for longer range engagements. (Which was probably true when used by the US Marines who still taught real rifleman shooting techniques. although the crappy burst mechanism goofed up the trigger pull for accurate shooting. ) The M-16A2 was greatly advertised as having a 800 meter effective range because that is how far the rear sight would go...When we went into Bosnia and the other Balkan countries all the locals asked about it... in disbelief.... So they employed scope bolt guns to counter us... BTW I have shot an A2 at 700 meters and about 30% of the bullets showed tumbling or yawing. I suspect ammo manufacturing and not the rifle.
I chronographed M-855 ammo from my A2 and it averaged 3,025 fps from the 20 inch barrel for the 62 grain boat-tail. When zeroed for 400 yards it will keep the bullet impact within 10 inches high or low out to 475 yards.
Of course the 14.5 inch barrel on the M-4 carbine.... now provides less velocity and less sight radius. Really making it into a 300 meter carbine. However on paper,,, a 62 grain M-855 round going 2,820 fps from a 14.5 inch M4 will still hit within 10 inches of the bore line out to 450 yards. *( when the carbine is zeroed at 380 yards) Although it is only going 1980 fps at that range. (like a 22 magnum at the muzzle)
On paper anyway... since Field Manuals have become the creations of the geeks squads who never deploy.
