There are some who say that the AR-15 and its direct impingement gas system is poorly suited to shorter barrel lengths. High Caliber Conversions, LLC laughs in the face of those people:

Some interesting things to note about this extreme(ly silly) SBR AR-15. First, the barrel has been cut down so far that a section of the gas block actually extends past it (it has to to anchor the gas tube with a roll pin). Second, the barrel is in fact shorter than the ammunition it is firing. 5.56mm has an overall length of 2.26″, and with a 2″ barrel nearly a quarter inch of the projectile is sticking out of the muzzle. This means the barrel has virtually no rifling length, and the projectiles start already exposed past the crown of the barrel; in the video the tips of the bullets can in fact be seen before each round is fired.

Third, the extremely short barrel means a large amount of wasted, unburnt propellant. This can actually be seen in the form of black smoke escaping from the ejection port during firing, and in certain frames coming out past the muzzle. It’s likely that due to a lack of significant engraving of the bullet in the rifling, much or even most of the propellant is not being burnt at all, resulting in fairly low muzzle flash, noise, and more modest recoil than one would expect with such an extremely short barrel. In some frames bright flash is evident, but it’s not clear how this would compare with that of other, longer-barreled SBRs.

An interesting but wholly impractical experiment in extremity, to be sure.

Thanks to Retiv for the tip.