The Small Arms Review, which is an awesome firearms magazine, has been running a series of interviews with various firearm designers, such as Arthur Miller, Reed Knight, Dr. Philip Dater, et al. The interviews are great, with lots of insights into designing firearms and making design tradeoffs. Unsurprisingly, many of these designers are connected to the M-16 system in one way or another, due to its prominence as the last successful American rifle design to have wide acceptance.
In OIF and OEF, there have been documented reports of unreliability with the M-16 and its derivatives, the M-4 and the Mk-18 CQBR. Soldiers' failure to clean their weapons contributes to some of the reliability problems. However, the rest of the reliability issues may be design-related, and several of the designers above discussed this topic with Small Arms Review in the interviews.
In particular, in the March, April, and May 2008 issues, Jim Sullivan talked of the design problems he dealt with in designing the M-16, and how they have contributed to the current reliability problems. Jim Sullivan was one of the lead designers to complete the AR-15/M-16 design. Mr. Sullivan said that the current M-4 reliability problems come from barrel heating. According to him, the 5.56mm cartridge, because of its straight case design, is very sensitive to barrel heating over the course of many shots. The chamber pressure from the gunpowder expands the case against the chamber of the barrel. As the case temperature increases, it pushes harder against the barrel chamber. Normally, there is enough time for the case to cool and relax (and depressurize) before case extraction starts, which pulls the case out of the chamber and ejects it. However, if the extraction starts earlier than designed, such as with the M-4 and other shorter variants, the case is still pushing against the chamber during extraction. This "sticking" to the chamber exerts a friction force against the extraction, which may be strong enough to cause weapon failures. During a long firefight or range session, the rifle barrel heats up over time. The barrel heating causes the chamber to expand itself, decreasing the clearance between the chamber and the cartridge case. This sticking process, according to Mr. Sullivan, is the cause of the M-4 reliability problems, and is a concern for all 5.56mm weapons with a barrel length of less than 16 inches. (The M-4 has a barrel length of 14.5 inches.)
The Russians avoids this problem entirely by using a highly tapered case for its 7.62x39mm and other rifle cartridges. As the case expands, it also pushes itself away from the chamber, due to its case taper. For the Russians, barrel heating and case heat improves extraction, and they can go with a shorter barrel for their compact carbines with no reliability issues.
For us Americans, however, we're stuck with the 5.56mm cartridge for the foreseeable future. The federal government's budget problems means there is no money to change our service cartridges. Therefore we need to find low-cost fixes for our M-4 carbines, which is coming into wide use in the US Army and the USMC. In fact, the Army is making the M-4 its standard soldier weapon, replacing the M-16. To deal with the chamber sticking problem, there are two main methods: increasing cycle length to give the cartridge case time to cool, or decrease barrel heating to draw heat out of the cartridge case.
One way to decrease barrel heating is to install a heat sink onto the chamber end of the barrel. The POF-USA piston operation system has a barrel nut that also doubles as the heat sink. The heat sink may contribute to the reliability of POF's weapons as much as its gas piston system. In addition, a non-free-floating rail forearm can work as the heat sink as well. [The link shows a free floating rail system. There aren't many non-free-floating railed forearms on the market. A non-free-floating forearm clamps onto the barrel, thus drawing heat, whereas a free floating forearm does not contact the barrel.]
The other approach is to increase cycle length, but Colt has already exploited this approach with the M-16 and the M-4 currently in service. Colt re-designed the buffers to slow down the bolt carrier during the ejection process. The modified buffers make the M-16 somewhat reliable, but is not an adequate solution for the M-4.
The AR-15 commercial market is debating the merits of the gas impingement system versus the piston/operating-rod system on improving AR-15/M-16 reliability. The piston/operating-rod system changes the force pushing on the bolt carrier, but as Mr. Sullivan says, that's not necessarily the solution. The root of the reliability problem comes from the barrel heating/cartridge sticking. This phenomenon is the result of prolonged rapid fire through the weapon, which is not how most users use their weapons. For most users, the current M-4 and CQBR configuration is adequate for their needs. For users who need to engage in prolonged rapid fire, though, they need to consider installing heat sinks on their barrels.
For the users who are looking for a short-barreled rifle/personal defense weapon, the cartridge sticking is a factor they need to consider in evaluating the market. As the barrel gets shorter, the problem becomes more pronounced.
In a later article I will evaluate the effectiveness of the Picatinny railed forearm as a heat sink.