The M27 / HK 417 is an AR15 with a gas piston retrofit. There is an advantage to piston operating systems for really short barrels, where DI systems do not exce

The M27 / HK 417 is an AR15 with a gas piston retrofit. There is an advantage to piston operating systems for really short barrels, where DI systems do not excel. In fact, many such piston AR retrofits exist in the US civilian AR market, but they’re generally thought to be of “unproven” longevity, at least. The advantages of the retrofit piston being slim and light is that the recoil increase is small. The disadvantage is that durability is questionable, as those pistons experience a lot of violent force and most guns designed from scratch to be piston operated have significantly heavier duty pistons, for good reason. You can read online of Marine armorers lamenting how their M27s are breaking down at a rate much faster than their M4s. This is of course anecdotal, but it is also not entirely worthless data.