M4 Carbine

The M4 Carbine, or just plain M4, is a shortened derivative of its predecessor and big brother, the M16. Both, the M4 and M16 are variants, derived from the first of the 2nd Generation Rifles to be used by the US Military, the AR-15 (Armalite Rifle #15) originally designed and created by Eugene Stoner and first manufactured by Armalite in the mid-1950s.

The M4 was designed to give soldiers a more compact, carbine sized rifle, capable of select fire with periods of long sustained full-auto firing, all in a standardized, lightweight platform that was familiar to users.

What's New?
As with any 2nd Gen Variant, major improvements were made that distinguish the M4 from the M16. Perhaps the most notable being the barrel length and shape, while the stock is the second most noticeable distinguishing feature of the M4. Other improvements and modifications consists of extended feed ramps cut into the M4's upper receiver as well as it's barrel extension that are often referred to as M4 Feed Ramps, by those in the industry. Another improvement made to the design and functionality of the M4's upper receiver is the flat top design that eliminates the carry handle and provides a picatinny railed surface for mountin optics and accessories to. Although this feature is not exclusive to the M4 design as some M16s can be found with flat top receivers, most incorporated the carry handle style upper receiver into their design. The barrel design received minor modifications besides those found in the barrel extension. Overall length of the barrel was shortened from the M16's standard 20" to 14.5" making the M4 ideal for use from within vehicles as well as inside buildings, since the shorter length allowed operators to clear doorways and narrow walkways while keeping the weapon shouldered, on target and ready to fire. Anotber modification to the M4's barrel design was the incorporation of a narrow band like indention located almost mid-way between the Front Sight Base (FSB) and the muzzle of the barrel. This indention allows for the M203 Grenade Launncher to rest from the barrel when attached to the underside of the weapons barrel. Due to the new collapsible stock design, the M4's buffer tube (often incorrectly referred to as a Stock Tube) was shortened and modified to accept the 4 position stock that comes standard on all M4s.

What Stayed the Same?


Besides the number of changes from the M16 design, the M4 does retain most of it's big brothers components andfeatures. For example, minus the extended feed ramps cut into the M4's upper receiver, both upper and lower receivers from the M4 and M16 are identical. Also, depending on the select fire functions the internal parts for both firearms are also identical.

Further Options
The main reason the same weapons platform was able to be utilized to create the M4 is because of the commercial or civilian versions of the M16 and M4, the AR-15. Thanks to the growing AR-15 parts industry there is an estimated 394.2 Million different possible variations of what is often referred to as the AR/M4/M16 weapon system. That's a lot of different possible part combinations. Capitalism has played the greatest role in the industry's significant growth. The Vietnam War was the first major operation that saw standardized use of the M16 weapon system. At this time, there were only a few manufacturers that were building M16s for the US Military. Sometime shortly after Desert Storm, it was estimated that there were at least 250 different manufacturers in the US that were producing either the select fire M16 or its semi-auto counterpart the AR-15. Over the next several years, as the popularity of this weapon system grew, so did the number of parts and accessories manufacturers. This increased the overall amount of money being used for R&D by 10 maybe 20 fold. Even today with the release of the first 3rd Generation Rifles, new modifications, improvements, parts and accessories are being made to the AR/M4/M16 platform.

Piston Driven Gas Systems
Still a fairly recent improvement to the platform is the addition of a Piston Driven Gas System. This style of an auto-loading mechanism is nothing new and in fact was being used on 1st Gen Auto-Loading rifles. On a standard AR/M4/M16, as the bullet travels down the barrel, being pushed by gases created from the explosion (which is in fact only black powder burning) within the bullet casing. Before the bullet leaves the barrel it passes by a small hole located mid-way. Some of the gases propelling the bullet forward escape up and out this hole to only force their way back toward the receiver through a tube that is barely an 1/8" in diameter, this is the gas tube. Once the gases clear the gas tube inside the upper receiver, they force the Bolt Carrier Group (BCG) back by way of the Gas Key mounted atop of the BCG. The force of the gases is so great that is pushes the BCG back into the Buffer Tube, positioned at the back of the upper receiver. Once the BCG reaches it's farthest point back and the gasses dissapate, the Buffer forces (under the power of the Buffer Spring, housed inside the Buffer Tube) the BCG forward, loading a new round into the chamber as it closes on the barrel extension. Once the trigger is pulled this process repeats itself, ejecting the spent casing and loading a new round. Each time these gasses are forced into the upper receiver, carbon fouling, black powder, and burnt residue collect on all the parts of the operating system. As this fouling builds up, jams and misfires can become more significant and time consuming. As you can see this can be life threatening in a combat situation.

The Piston Driven Gas System replaces the gas tube with a spring loaded piston. Everything remains the same, except when the gasses escape up out of the small hole located atop of the barrel, they force a piston backwards into the upper receiver that then forces the BCG back into the Buffer Tube. This method allows excess gasses to dissapate out the top of the barrel, mid-way where the gas port is located instead of inside of the Upper Reciever where the BCG is. As the extractor, barrel lugs, and firing pin are all located on the BCG, this system does not allow any fouling to reach the upper reciever nor the lower reciever's trigger group. This keeps all the parts within the receiver clean and running cool for much more prolonged periods of time. The Piston Driven Gas System also slows felt recoil making it ideal for any AR/M4/M16 weapon system but even more so on select fire weapons that will see sustained full auto fire.

A few really good aftermarket parts manufacturers have designed and built various Piston Driven Gas Systems to allow currently issued weapons to be quickly and easily adapted from the Direct Impingement Gas Driven System to the Piston Driven Gas System further enhancing reliability and functionality. It is worth noting that almost every AR/M4/M16 manufacturer now offers at least one if not two Piston Driven rifles in their product lineup. Also, all currently marketed 3rd Gen Rifles run on a Piston Driven Gas System as this seems to be the most reliable and accurate method for working the action in auto-loading weapon systems. I should also mention that the SKS and it's little brother, the AK-47 and it's variants, all function on a Piston Driven Gas System.

Polymer and Carbon Fiber Receivers
All M4 receivers are currently manufacutered from either forgings or billet pieces of T-7075 Aluminum which is 60% stronger than the early M16 receivers that were once produced from T-6065 Aluminum. Bushmaster started a line of AR-15 Pistol's a few years ago that were built on Carbon Fiber Upper and Lower Recievers. This allowed for significant reductions in weight. As of mid-2013 you can purchase lowers and upper's from various manufacturers made of Polymers similar to a Glock Pistol or Carbon Fiber, both of which are strong and lightweight materials. Since using these materials in production is still relatively new, it is hard to say how long one should expect a Polymer or Carbon Fiber M4 to last. It is however, a safe bet to say that under sustained full-auto fire, a non-metal Upper Reciever will wear significantly faster than one made from Aluminum. If you are looking to build the lightest M4 possible, then a Polymer or Carbon Fiber receiver is the way to go.

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