At the Horus Heresy Weekender there was an open game in the evening called 'Last Man Standing'. I missed it because it was late on and I wasn't staying at the hotel but I took a snap of the rules (see below). It's basically a death match between X number of players, each of whom takes control of a single basic Tactical Space Marine from one of the founding legions. They can be equipped with any of the optional s available to a tactical marine (including heavy/special but not sergeant only upgrades like plasma pistols).
The choice of legion is important because each one has a different effect on the basic stat line, for instance Death Guard get +1 Toughness whilst Space Wolves get +1 Attack. So think carefully about your style of play before you enter the arena!