I'm not an expert in the differences in barrel length's when it comes to AR's, but it seems to have been narrowed down somewhere between 14.5" and 18". Personally I would say 16", just seems to be a more common barrel size and leaves a lot more options open, as well as things like hand guards and such, although I can't comment on the ups or downs of certain gas systems. Some other tips I would offer though it looking at things like brand, price point, twist rate and what he plans to use it for.
Of course, as it has already been mentioned, Plan B can always simply just to have the lower registered and get two or three separate uppers that he can switch out.
Of course, as it has already been mentioned, Plan B can always simply just to have the lower registered and get two or three separate uppers that he can switch out.