Most site I've played at have used the front sight + rear sight + target rule, added with the A4 paper size rule, and no opening or closing windows.
I played a site last year where i was crouched behind a small wall. A walk on then started poking his gun round the side and spraying (luckily nowhere near my general direction as i was tucked in). Only thing i could do was grab the barrel and pull the gun away. When he then came round the corner to find out what happened he had no idea i was there and walked into a knife kill