I first saw the "dissipator" style rifles in Bushmaster catalogs back around 2002 or so. They listed them as a 16" barrel with a rifle length gas system. This, in theory, should lead to a softer shooting rifle with the advantage of a full rifle length sight radius. This was before the true advent and popularity of full length free floating handguards over low profile gas blocks. It can still be done, but a mid-length gas system with a full rail will give you a similar effect. If you want standard rifle length hand guards on a 16" barrel though, it is still an option. I think the look is the only thing keeping it from becoming popular, as I have seen no downside and the additional sight radius is almost always a good option.