[magila]

I'm not sure what you're getting at. The AR-18 was intended as a cost-reduced substitute for the AR-15, hence the change to a stamped steel receiver instead of forged aluminum. Design wise the AR-18 is still more closely related to the AR-15 than the AK-47, although it is significantly different from both.

[darkpuma]

The AR-18 was in fact a technically inferior design. In an AR-15/M-16, the big deal with the "direct impingement"[0] system is that all the reciprocating mass is in line with the bore axis, which avoids imparting any angular moment into the rifle making it easier to keep on target when shooting rapidly. The AR-18 throws that away by having reciprocating mass (the short-stroke piston) above the barrel.

And "shits were it eats" jokes aside, the 'DI' system of the AR-15/M-16 is very reliable. If you look at close in high speed footage of AR's and AKs being fired, you'll see that both rifle designs vent a lot of combustion gasses into the receiver when the cartridge is ejected, so the cleanliness of an AK compared to an AR is somewhat exaggerated.

[0] Which is in fact a piston, with the 'piston' being the rear of the bolt, and the bolt carrier forming the cylinder; driven apart when the hot combustion gasses are piped in between them. But this is a controversial nitpick.