I've been starting to think about building an AR upper around some of my own peculiarities and quirks for home-defense, so I thought I'd throw the concept out for discussion and see what potential pitfalls you guys see. General note: I like my weapons more muzzle-heavy than normal, it may cost a little more weight but helps with recoil. Two questions before starting: 1. Is there a problem with having a forearm extend past the muzzle and around the flash-suppressor? 2. Are rifle-length gas systems not used on 14.5" and 16" barrels because of gas pressures, or because the rifle-length gas port is too close to or past those muzzles? Concept starts with a 14.5" barrel and a permanently-pinned flash-hider to bring total barrel to at least 16.01", with the longest gas-system that will fit. Whether monolithic-upper or flattop-plus-quad, top rail as close to either the muzzle or the end of the flash-hider as possible--I want every last possible millimeter of sight radius I can get out of the upper's length, while keeping it barely above legal limit. We're talking a home-defense instrument for Bad Breath Ranges in an NFA-hostile area, and quarters are so tight that a person above about 300-325# completely fills the entire hallway, and in some places even has to turn sideways to get through, so the goal is something very maneuverable and quick into action.