In a pair of posts late last year (Part 1, Part 2), this blog covered several arguments surrounding the American military’s primary rifles, the M-16 and M-4, which are also widely used by police departments, counterterrorism units and other federal agencies. (Among those using M-4 rifles are the C.I.A., the State Department and the New York Police Department.)

The posts were prompted by the leak of an official historian’s account of the battle of Wanat in 2008, in which a remote Army outpost in eastern Afghanistan was nearly overrun by insurgents. Nine American soldiers were killed that day. The historian’s draft study suggested that at least a few American weapons failed in the fight, perhaps because of overheating.

The reports of weapons’ stoppages in combat resurrected longstanding controversies surrounding the M-16 line.

Some of the arguments (the problems with M-16s jamming during their Vietnam-era rollout) were historical and of marginal relevance to the current generation of rifles. The causes of jamming in the jungle and deltas of Vietnam were isolated more than 40 years ago, and the rifles and ammunition in use today are significantly changed. Complaints from Afghanistan and Iraq are also on a far smaller scale than those of yesteryear, when the early generation of the M-16 became a national scandal.

But other arguments remain germane, including the question of whether the rifles are susceptible to stoppages caused by the intrusion of sand and dust, and whether the standard military ammunition they are firing is well suited for killing lightly clad men.

The current round, the M855, was designed for penetrating Eastern Bloc body armor. Some soldiers believe it passes too easily through victims wearing everyday clothes.

There is also a camp – and when discussions turn to infantry arms there are many camps, with many motivations — that claims the M-4, which has a shorter barrel than the M-16, does not fire bullets with enough muzzle velocity to maximize soft-tissue damage in a struck man. (I mentioned in a previous post that I will discuss arguments about bullet composition, barrel length and wounding potential in a future post, and I will. I will also look at the questions and test results related to stoppages caused by dust and sand.)

But now it is time to turn back to a question that followed Wanat: whether the rifles are prone to failures caused by overheating in intensive combat, when a soldier or a Marine might fire multiple 30-round magazines in rapid succession. This causes heat buildup in the barrel that can extend down the rifle to the weapon’s guts, and — in theory if more than in practice – make a rifle too hot even to hold.

Next week we will post a pair of videos of post-Wanat M-4 heat testing, and discuss a recommendation made to the Army by the manufacturer to address heat-buildup concerns. In the interim, via the photographer Todd Heisler, the blog offers an unusual look inside the plant where almost all M-4s and M-16s are made.

The two rifles are principally manufactured by Colt Defense L.L.C., of Connecticut, the descendant of the company that sold the original M-16s to the United States in the 1960s. That company in turn descended from the original Colt Patent Fire-Arms Manufacturing Company, the brainchild of Samuel Colt, who in the 19th century ushered the revolver into common use with a factory in New Jersey and then Hartford. The Hartford plant later produced Gatling Guns and Thompson submachine guns under contract, and manufactured the M-1911 .45-caliber pistol and machine guns under its own name.

The military firearms business is prone to booms and busts linked to the cycles of war and military expansions and contractions. Colt’s many incarnations have flirted with insolvency several times. The M-16 line has been its savior. (A sister company manufactures civilian firearms, and Colt Defense has been expanding business to include manufacturing spare barrels for the M-240 machine gun used by the American military.)

The company’s latest incarnation is run by a retired Marine Corps general, William M. Keys, and privately owned by about 30 different shareholders, with the bulk of the shares held by Sciens Capital Management, an equity group in New York. It operates a factory and small corporate office in West Hartford, Conn., where it employs about 800 workers, most of whom are members of the United Auto Workers union.

A decade ago, the plant made as few as 150 rifles a week, General Keys said in an interview on Tuesday. It now manufactures 4,000 rifles weekly, many for the United States military but others for police agencies and international customers, including Mexico, Malaysia and India. The factory runs full production from Monday to Friday, with a two-week shutdown each summer and again around Christmas and New Year’s. (The factory restarted production on Monday after a seasonal break.)

For bulk purchasers, a new M-4 costs about $800 per rifle, though the price is often higher when after-market rail systems, used for mounting optics and lights, are included. For rifles used by the American military, the United States Army requires Colt to install a rail system manufactured by Knight’s Armament Company of Vero Beach, Fla.; this pushes the price per piece to about $1,100.

The photographs above offer a feel of the plant and show some of the processes of machining and assembly.

Several steps occur at other companies. Colt has no foundry or injection-molding shop, for instance. It purchases the sleeves of steel from which it manufactures barrels, and it subcontracts for the aluminum casts that, after machining inside the plant, are ground and cut for many of the rifle’s parts.

Similarly, it buys its rifles’ hand-guards and stocks. The rifles’ phosphate protective coating, which gives the metal parts their non-corrosive black finish, is also outsourced.

But the main operating parts are all machined here, and final assembly occurs in one corner of the plant, where the rifles come together and then are subjected to inspections and firing tests, including by government inspectors who work full-time on-site.

Controversies over what rifle is best for American military use continue. In all likelihood, they always will. Infantry rifle selection has always been about compromises, and arguments of which rifle, caliber and bullet composition are ideal for combat are a perennial infantryman’s dispute. But for now, Colt Defense has the market.

(Todd Heisler’s photographs in today’s blog post show the origins in New England of almost all of the American service rifles seen in the footage and images from Afghanistan and Iraq.)