The U.S. Marine Corps has ordered another 50,000 M27 IAR (Infantry Automatic Rifle) to see if these would be an effective replacement for M-16 type rifles (like the M-4) in combat units. The M27 costs about three times more than M16 or M4 rifle and is a little heavier. But marines who have used both in combat tend to prefer the M27. This was especially true for marines with many years of combat experience. Some marines have used M27s, on and off, for years.
Widespread M27 use was the result of the marines buying about 15,200 since 2009 as light machine-guns to replace the M249s long used in marine infantry battalions. These battalions were allowed to retain some of the M249s, to give the battalion some options. The marines withdrew from service about 20 percent of their 10,000 M249s once the first 4,200 M27s were delivered. Marines, and infantry in general have always preferred having more automatic weapons. Having some additional light machine-guns (like the M249) in reserve and available for emergency situations has been a marine custom since World War II. But at this point some marines began to note that the M27 was a big improvement on the M4, as well as the M249.
The M27s began entering regular service in 2011 and as more were used in combat it became apparent that the IAR was very effective and popular with its users. In late 2016 the marines experimented with equipping all the riflemen in a battalion with M27s for large scale training exercises and were impressed by the results. That, plus the excellent combat record, led to the willingness to spend over $30 million to buy a lot more. The M27 costs $3,000 each compared to $750 for the standard M4 assault rifle (s variant of the M16 used by the army and marines since the 1960s.)
The M27 is a 3.6 kg (7.9 pound, empty) automatic weapon based on the German HK416. It has a forward grip, a bipod and heavier barrel and can use a 30 or 100 round magazine. Unlike the M249, it does not have an easily replaceable barrel, but it is more accurate and has a slower rate of fire (560-640 rounds per minute). The M27 uses a mechanical system that is less likely to jam, as well as a floating barrel (for better accuracy.) Marines found they could use fewer rounds of more accurate fire with the M27 than they did with the M249. The M27 can use larger magazines but required a skilled and discipline user to prevent barrel overheating. In other words, you only go full automatic if it is necessary. With modern sights and other accessories it is easier to do that, especially 5.56mm weapon that is actually more accurate at longer ranges than the M4 or M249.
The M27 was the result of five years of research and development to create a weapon that would replace the M249, which the army and marines began using in the early 1980s. The marines have had a lot of complaints about the M249 in Iraq (jams from all the dust and sand), and many of the marine M249s are simply wearing out.
The marines were originally looking for an IAR that weighed between 4.8 kg (10.5 pounds) and 5.7 kg (12.5 pounds) empty, used a large magazine (100 rounds or more) as well as the standard M-16 30 round magazine. The heavy barrel on the IAR had to be able to handle sustained fire of 36-75 single shots a minute. The higher number was the ideal. It had to have the standard rail on top for mounting accessories, be resistant to jamming from dust and sand and, in general, be a lot better than the M249. The marines always planned to buy 4,000 weapons initially, and wanted to do so as soon as possible. The marines later sought a manufacturer who could produce the 11,000 additional M27s as quickly as possible because marines in combat made it clear they preferred the IAR. .
U.S. Army Special Forces pioneered the development of the 5.56mm light machine-gun during the 1960s, when they obtained the first experimental models for use in Vietnam. The Special Forces and SEALs were very impressed with the light weight, and heavy firepower, from these weapons. But it took over a decade for the regular army to adopt such a weapon, mainly in response to the success the Russians were having with their own version of the lightweight squad machine-gun. The M27 was a deliberate effort to develop something to fill a niche between the M-4 and the M249.
The army has also expressed interest in an M249 replacement, and have followed the marine search with great interest but have not yet acted. The marines finally selected the lighter HK M27 entry after three months of field testing. The M27 is quite similar to the first American "light machine gun", the M1918 BAR (Browning Automatic Rifle). This was a 7.62mm, 7.25 kg (16 pound) automatic rifle, similar in appearance to the later (1950s), and lighter M-14. The BAR was revolutionary in World War I, and was used by the United States until the 1960s, when it was replaced with 6.6 kg M-14A1s equipped with a heavier barrel. This was not a satisfactory solution, and led to the development of the M249 and the M27.