This article is an excerpt from the Gun Digest Book of Classic Combat Rifles.
Chrome-lining the barrels gave three major improvements to the standard barrel. First, the chrome-lined barrel was corrosion resistant. Second, chrome is slippery in nature and assists in extraction and ejection. When chromed, the walls of the chamber are harder; sand and mud don’t “iron” into them. Thirdly, chrome is 2 to 3 times harder than standard barrel steel so the barrel lasts significantly longer.
The new, improved M16/M16A1 barrel assemblies would have stamped on the barrel, in front of the front sight assembly: “C” (Chrome Chamber Only), “C MP B” (Chrome Chamber, Barrel & Magnetic Resonance Tested) or “C MP Chrome Bore”(Chrome Chamber, Barrel & Magnetic Resonance Tested). Many experts, including Bill Davis, felt the failure to chrome the chamber was responsible for many of the early malfunctions in Vietnam.
The flash hider was changed from the early three-prong to the new “bird cage” style. The three-prong suppressor was superior to the new design, but was snagprone in the field. With these modifications in place, the M16/M16A1 rifle was “perfected” and performing to the Department of Defense acceptance standards.
The AR15/M16 Carbines
Soon there was a demand for a smaller, more compact, version of the rifle. Early in 1966, the Army expressed interest for a carbine for its special operation units, placing an order totaling some 2,050 carbines. Lieutenant Col. Yout later ordered an additional 765 Colt “Commandos” — and a new name was coined for the carbine project. The first carbines were known as CAR15 (Colt Automatic Rifle). These first designs incorporated a 10-inch barrel and a sliding butt stock. Later the barrel was changed to 11.4 inches to permit the weapon to launch grenades. The Army signed a contract for 2,815 “Commando model” submachine guns on June 28, 1966.
As expected, the CAR15 — now the XM177E2 — successfully passed all testing phases at Aberdeen Proving Ground. However, a new problem appeared: the deafening noise and large fireball from the muzzle, thanks to the CAR15’s higher cyclic rate of 700 to 1,000 rounds per minute. As a remedy, many of these rifles were equipped with 14.5-inch barrels, a practice that carried over to the M4 project of the early 1980s.
Product Improvement (PIP)
On October 28, 1980, there was a new 5.56×45mm cartridge on the block. NATO (Northern Atlantic Treaty Organization) had adopted the Belgian-made SS109. This new bullet had two major differences from the GI 5.56×45mm M193 Ball cartridge. First, the bullet weighed 62 grains instead of 55 grains. Second, this new bullet had a hardened steel penetrator core, giving this new 5.56×45mm round better penetration at all distances than the 7.62×51mm NATO (M80 Ball) round.This new SS109 round penetrated three 3.5mm mild steel plates at 640 meters and a U.S. issue helmet at 1,300 meters.
The new 5.56×45mmNATO round revolutionized military small arms ammunition all over the world. In 1974, the Soviet Union switched from the 7.62×39mm (AK47/AKM) to the 5.45×39mm Soviet round of the new AK74 rifle. This new round was a .221-inch diameter 52-grain full metal jacket boat-tail armor-piercing bullet with a velocity of 3000fps.
The new SS109 round was more lethal than the original M193 Ball round due to the faster “spin” and fragmentation upon impact with soft tissue.