Considerable confusion and a seeming decline in accuracy have accompanied the adoption of the heavier-barreled M16A2. Adding to the confusion has been adoption of the Belgian SS-109 62-grain bullet as the U.S. M855 round, which replaces the original lightweight 55-grain M193.
We'll sort through this below, but keep one thing clearly in mind: use the original, lightweight 55-grain round in the original, lightweight M16/AR-15 and performance will be fine. Likewise, fire the heavier round with the heavier M16A2 to avoid problems. Simple enough.
Now here's the explanation. Although it took two decades to face it, the U.S. military finally recognized that the 5.56mm round lacked sufficient energy and penetration for midrange shooting, The M16A1 was considered to have a maximum effective range of only 460 yards.
The solution was immediately apparent:
increase bullet weight by 10 percent and you'll increase performance by 10 percent. The Belgians already had perfected such a bullet, which proved to fit the bill perfectly. However, because this bullet was heavier and a hair longer (4/10 of a centimeter) than the 55-grain round, it did not stabilize properly in the M16A1 barrel, which has 1:12 rifling.
How bad was this mismatch? In U.S. Army tests, a six-round, 100-yard group firing the heavy Belgian-designed bullets through an M16A1 measured 12 inches across—12 inches! When the proper 55-grain ammo was fired through the same rifle, that group instantly tightened up to 3 inches.
It was found that the heavier, longer, 62-grain rounds worked best when fired through a barrel having very fast 1:7 rifling, which is the rifling used in the M16A2.
Keep this sorted out and you shouldn't have any problems.
Though the 175-grain M118 Long Range cartridge has pushed an additional 100 or so yards of effective range out of the .308, realistically this round runs out of a lot of "oomph" after flying 1,000 yards. By then it has slowed to only 1,200 fps, generating 560 ft-lbs. of energy, only about half of what it possessed at 600 yards, which reduces its penetration and lethality. Further, its declining trajectory has become such a sharp arc that even a minor range-estimation error can cause a clean miss.
There is a way to boost that long-range performance and attain more effective velocity and terminal effects at 900 to 1,100 yards—by employing the cartridge king of long-range shooting, the .300 Winchester Magnum. This powerful cartridge traditionally has dominated ,30-caliber, 1,000-yard shooting at the NRA's annual Camp Perry matches. It was because of the .300 WinMag's impressive performance that the US. Army's M24 Sniper Weapon System was designed to be readily convertible from 7.62mm to the heavier round.
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