Lockedon,
the AR-18 was developed to be manufactured by third world countries. In the early 60's, the Soviet Union, the US and allies were practically giving away AK-47's, M-14's, etc., to any country that would take them so there was no foreign market for the AR-18.
The US military ran tests at more than one Army base. I was fortunate enough to meet a former Army sergeant who was part of the tests done at Fort Knox. He told me the troops doing the actual testing loved the weapon but the tests were altered further up the chain of command to reflect an inadequate weapon.
Today, one current AR-15 type weapons manufacturer uses a portion of the AR-18/AR-180 gas system.
Quick history: The forerunner of the AR-18 was the AR-16 in 7.62 designed by Eugene Stoner. The AR-18 was designed in 1963 by ArmaLite's ART Miller.
The AR-180 was manufactured by:
ArmaLite, Costa Mesa
Howa, Japan (Under license from ArmaLite)
Sterling, England (Under license from ArmaLite)
From all three manufacturers, about 20,000 AR-180's were made.
I've owned AR-180's made by all three manufacturers and found them, functionally, about the same.
5sub
P.S. I only know one person who has all the data and knowledge to make a legitimate side-by-side comparison and I doubt that he would be willing.