Quoted: Doesn't it potentially affect the gas system, because there is less distance between gas tube/block and end of barrel, therefore less amount of gas to blow back bolt?
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It's about the same amount as on a 20" rifle, which the whole system was originally designed around, and it's farther up the tube so the pressure is higher. What you're worried about is NOT a problem. In fact, you have more problems going the other way, down to carbine length, as the gas is ported even higher up the pressure curb with more barrel left to go. This has caused problems, nearly all of which have eventually been worked out more or less satisfactorily over the years, to the point that the carbine/M4 length system is now fairly reliable. But the midlength is still just a better mousetrap all the way around in a 16" barrel.
Then there is the so-called "dissipator" length. More sight radius, but would seem to have reliability problems, although most of the guys who have them report that they work very well, too.
I like my midlength for a number of reasons, among them looks and handling/feel. I wish it had the ballistics and sound level of a 20" barrel, but then that's what my 20" guns are for, right?