They could buy some of the very fine piston operated models. I was never happy with my M16, as it was pretty good through the first 3 mags, then got hot and unreliable. I believe that the gas piston on like the POF model would solve the problem. My next AR will be gas piston. (I know, lots of controversy on this issue).
Like I said, get a mid-length upper and many of your problems will be solved.