Brownells Gun Techs Steve Ostrem and ‪@CalebSavant‬ are back to take on the myth that piston-operated AR-15s are better / more reliable than standard direct-impingement ARs. As the guys demonstrated in an earlier Smyth Busters episode ("Is the AR-15 Really Piston Operated?"), the AR-15 is not actually a pure direct impingement rifle but a kind of hybrid, with the bolt operating like a piston. In this episode, "direct-impingement" means an AR-15 with the traditional gas tube above the barrel.
Caleb points out there have been a lot of attempts at piston-driven AR-15s, some of which were terrible. The Heckler & Koch HK416 was purpose-designed to be piston-operated, and it works VERY well, as does the Brownells BRN-4. Any good piston-operated AR-15 style rifle will be more expensive than a direct impingement one because it just costs more to manufacture the additional moving parts. And it is heavier.
So what's the benefit of a piston? A piston AR runs CLEANER and COOLER. It doesn't blast hot gas back into the receiver and bolt-carrier group, which increases the gun's longevity. This is a huge benefit in full-auto fire, too. A direct impingement AR-15 runs hotter and dirtier, but the rifle is lighter. And after 60+ years of development and refinement, it is also very reliable.
What about those "plug-n-play" piston conversion kits? They involve engineering compromises that cause "bolt tilt." In direct-impingement, the gas pushes on the bolt in line with the bore so the bolt and carrier move in a straight line. With a piston conversion, the force hits a lug on top of the carrier where the gas key would be. This force is off-center, causing the carrier to tip down at the rear, tilting the bolt upward. The result is accelerated bolt wear and malfunctions. Purpose-built piston guns like the HK416, BRN-4, and AK series are designed to keep the piston force properly centered.
So the myth is BUSTED. A piston AR is not always better than a direct impingement AR-15. Direct impingement is more reliable than a piston conversion kit. But a purposed-designed piston system is more reliable than either of them. A direct impingement AR-15 is the most cost-effective option for sport shooters. But if you have the spondulicks, try the HK416 / BRN-4 route.