M4 Carbine: Why It Took 30 Years to Field a Shortened M16

The M16 Rifle Was Adopted In The 1960s; The Carbine Version Didn't Come Until 1999

The M4 Carbine is an iconic U.S. military weapon — and yet, the general public probably can’t distinguish one from an M16. And that’s fair, because it is an M16, but also, it isn’t. The venerable M4 has an odd history for a military firearm that began as a special modification to a full-length rifle that ended up replacing that rifle in its role as a primary general-issue weapon. 

But this story isn’t just an American military story. The M4 has been adopted by more than 60 countries throughout the world and has been manufactured by a bevy of companies, including Colt, Remington, and FN Herstal. 

So what is the M4 Carbine, how did it come into existence, what did it do that the M16 couldn’t, and what firearm is the military currently replacing it with? Let’s find out. 

M4 Carbine - The Design

The M4, like its parent AR-15/M16, is a lightweight, gas-operated, detachable box magazine-fed, air-cooled, select fire carbine chambered in 5.56x45mm NATO. As such, it has most of the features of an AR-pattern firearm: an internal piston (direct impingement but not exactly), a rotating bolt, a reciprocating bolt carrier, and a two-part aluminum receiver. In fact, the M4 shares 80% of its parts with the M16A2 rifle.

The differences lie in the M4’s shorter 14.5-inch barrel compared to the 20-inch M16 standard barrel, the accompanying shorter gas system and handguard, and a shorter receiver extension and buffer. If you get into building ARs with affordable parts, you learn those differences real quick. 

Early M4s had the same integrated carry handle as the M16, but later versions came with a flat-top receiver and a detachable carry handle mounted on a Picatinny rail that could be easily used to mount optics. 

 The furniture — buttstock, grip, and handguard — are made of reinforced plastic and the receiver extension (buffer tube) can accommodate a fixed A2-style stock, a law enforcement tactical stock, or a multi-position telescoping stock. 

The fire control unit on the OG M4 Carbine had safe, semi-auto, and three-round burst settings. The M4A1 replaced the burst with a full-auto setting. If you happen to have an opportunity to go through a bunch of military surplus M4s from different sources and eras, you’ll see that some have a heavier profile barrel, sometimes called a SOCOM profile barrel. The idea was that it could better withstand sustained full-auto fire. Oddly enough, SOCOM operators had moved toward using the lighter “government profile” barrels paired with a mid-length gas system. 

The Advantages Of The M4 Carbine

You might think, “So, what’s the big deal, it’s a short-barreled M16. Lots of people have SBRs.” It really is that simple. While civilian AR users along with law enforcement have discovered that short-barreled ARs can offer a great balance of maneuverability and accuracy by getting plenty out of the 5.56 or .223 round to matter at combat distances. 

But things move slowly in the military, and technically, the broad need for a standardized carbine didn’t become ultra apparent until the Global War on Terror (GWOT) began in full force. 

Countries that adopted a rifle platform that isn’t an AR kind of have it easy. If they need a shorter “paratrooper” version of a rifle, they just cut the stock off, put a short folding stock on, and maybe cut down the barrel in front of the gas tube. Well, an AR has its buffer tube in the stock, which makes a short stock possible, but makes a folding stock impossible without further mods. And, there really isn’t much barrel in front of the front sight post/gas block on an M16 to cut off — and you still need a muzzle device. 

However, the need for a primary firearm chambered for a rifle cartridge that’s only a bit bigger than a submachine gun with a stock has always been there. 

The Road From The M16 To The M4 Carbine

When the military began to adopt the M16 in the 1960s, a shortened version was almost immediately desired by some troops in Vietnam and that call was answered by Colt’s CAR-15 family of firearms. These weren’t general issue carbines and many of them had some issues because they just hadn’t found their ideal form yet. 

The CAR-15 barrel was 10 inches long; that’s pretty short and it played hell with the 5.56 ballistics. Range and accuracy dropped and the short barrel produced a large muzzle flash, which had to be tamed with a significant flash hider, which was like taking a step backward on the whole shortened barrel idea. 

They were issued in limited numbers throughout the war and with heavy use seen by MAC-V SOG units and other special operations operators, though they often modified their own firearms pretty heavily. 

But the need in combat had always been there. The reason the M16 was created and adopted is part of it. Most combat engagements don’t occur at significant distances, especially in urban and mechanized warfare, and in a large percentage of jungle combat. When the M1 Garand chambered in .30-06 was the standard issue battle rifle, firearms like the M1 Carbine and the M3 submachine gun (Grease Gun) were widely issued and widely used, despite the fact that the .30 Carbine cartridge is widely considered underpowered.

These guns were more maneuverable, easier to control, had larger ammo capacities than the Garand, and the Grease Gun was actually useful in full-auto. They were also lighter and more rounds of ammo could be carried. In short, these shorter weapons proved more useful in many cases than a full-sized rifle. 

So why did this role seemingly get forgotten by the military? It didn’t — it was just that the goals the military wanted the replacement for the Garand to reach were a bit out there for the time. 

They wanted one weapon that would take the place of the Garand, the M1 Carbine, and a submachine gun. Originally, and somewhat laughably, this was supposed to be the M14. It was chambered in .308 instead of .30-06, letting it use a short action, making it smaller than the Garand while still capable of long-range fire with accuracy. It was also select fire, which was intended to fill the submachine gun role. Engage a target at a distance, close that distance, and use the same firearm for mid-range engagement, and then, if clearing a trench, flip the switch to full-auto. 

Things didn’t exactly work out that way. The action on an M14 is shorter, but at 44 inches and change, the rifle is actually a bit longer overall than the M1 Garand at 43.6 inches. So it was not more maneuverable nor did it work well with tight ground vehicles or helicopters. 

We already talked about the usefulness of a long-range weapon in combat. And while the .308 has less felt recoil than the .30-06, it still kicks and is largely uncontrollable in full-auto. Besides, the M14 only had a 20-round magazine. 

It essentially failed in all those intended roles except the long-range engagement task, which is why the M14 evolved into a designated marksman rifle. 

So, the M16 was meant to do all of that but better, and for the most part, it did. The smaller .223 cartridge was found to be more controllable in full auto with training, but the M16, M16A1, and the later M16A2 were still full-length rifles, and while they were relatively lightweight, they were still cumbersome to use in conjunction with vehicles and in close-quarters battle (CQB) or close-quarters combat (CQC) situations. Some submachine guns were consequently issued to tank crews and the like, but most soldiers, Marines, and sailors had to make due. 

Finally, A Formal Request

A formal request for a carbine version of the M16 didn’t come until 1982, and by then, the military was using the M16A2, which was built on the Colt 645. U.S. Army Armament Munitions Chemical Command worked with Colt to develop a new variant labeled the XM177E2, which the U.S. Army called the XM4 Carbine, and later, the M4 Carbine. I could never tell if it was meant to be a successor to the M3 Carbine, which was the final iteration of the legendary WWII-era M1 Carbine, or the M3 Grease Gun.

But for the most part, it was peacetime, and things moved slowly. The Army began testing the XM4 in 1983. It was tweaked a bit with new furniture and a change in rifling. A barrel length of 14.5 inches was settled on and in January 1984 the Army approved the development of the new carbine. Funding for production waxed and waned until the XM4 was finally finished in 1987. The U.S. Marines were the first to adopt it in limited numbers. 

But it seems its usefulness still needed proving. The Gulf War in 1991 supported the usefulness of the M4 and Colt got its first production contracts in 1993, along with a contract for tweaked M4A1 carbines for SOCOM operators. This was spurred by information gathered from the Battle of Mogadishu in 1993. Army Rangers reported that their M16s were unwieldy in the urban combat environment while being transported by cramped Humvees, while the Delta Force operators had a much easier time with their CAR-15s.  

The M4 Carbine saw its first battlefield deployment in Kosovo in 1999 when U.S. troops supported the NATO-led Kosovo Force. 

While the GWOT began with most troops on the ground carrying M16s, during the course of the 20 years of operations in Iraq, Afghanistan, and elsewhere, the M4 largely replaced the M16A2, especially in the Army, as the primary weapon for forward-deployed units, while the full-length rifles were relegated to support personnel use. 

Because of its ease of carry, the M4 Carbine has also proven ideal for non-infantry troops and has gone on to replace submachine guns and pistols in many cases because it fires a more effective round — especially against body armor, and is as compact as is needed for most duties. Tank crews, for example, don’t need a special cut-down rifle or a submachine gun with different mags and ammo — they can just use an M4 and the same ammo and mags as everyone else.

Currently, the army and USMC use only the M4 Carbine and the M16A4. The Air Force issues M4s to its Security Forces, but other personnel use an M16A2. The U.S. Navy uses M4A1s for special operations and vehicle crews. 

The Marines were always the strongest proponents of the M4 and the first to jump into issuing them in large numbers. In 2009, the Army took control of the M4 design and allowed companies other than Colt to jump in and changes were presented and tested. 

Yes, with the shorter barrel, the M4 has a shorter effective range than an M16, but again, the need for that reach is limited, and there are still M16s and other rifles for when they do. 

But it would appear the M4 Carbine’s time is limited. In 2022, the Army selected the XM7 rifle, built on the SIG Sauer MCX Spear platform, was selected as the “Next Generation Squad Weapon” to replace the M4 and M16.