For ‘mouse gun’ enthusiasts tired of the temperamental, even fastidious, maintenence requirements of direct impingement guns such as the AR15, I have a suggestion.
Consider the Daewoo DR200 in .223/5.56mm. It has the upper/lower construction of the AR and the gas system of the AK74.
For a reliable gun, suitable for enemies over three feet tall, consider the Daewoo DR300 in 7.62X39.
Unfortunately, the Pentagon didn’t.
I have one and it's sweet. Takes any AR magazine. About $500.