Hi, Sam Colt,
I did not mean that gas piston system in general was not developed, I meant that the AR-18 was not a fully developed rifle. Its folding stock was not well designed and was a weakness, plus the failure to capture the bolt springs on the guide rods when the rifle was disassembled was not a good design; a direct copy of the M3A1 system would have been better. The AR-18 is easier to field strip and clean than the AR-15, and it has no spring in the butt stock to collect water or moisture. In addition, its guide rod bolt system, as opposed to the tight bolt and bolt carrier fit of the AR-15, allows plenty of room for dirt and gunk that would otherwise stop the rifle.
Overall, the rifle, I thought, was better than the AR-15, not only because of the piston system but because of the economy of manufacture and consequent cost savings. It could also be made in countries with a less developed manufacturing base, a benefit for U.S. allies who could manufacture their own rifles rather than having the U.S. supply them at a grand a pop.