I still think Colt's 6520/6530/6720 pencil barrel is the light weight general purpose barrel to have. People talk about the chrome lining hampering accuracy and the carbine gas system outdated but they shoot good enough and are reliable to the end.
And having owned a few mid lengths and even more Colt pencil barrels, the difference just ain't there.
I had the same M16A1 for two years in an Inf unit. I did a mag dump through it at every available opportunity like every other 19yo would. It was a Gen Motors Hydramatic, not even a Colt, but it had that same weight barrel as the SP1, 20in long. That rifle would flat shoot. I could drive nails with it. Lord only knows how many rounds it had seen before I ever got my hands on it.
