Virtually any inertia driven system Benelli has. If you load it, it fires without adjustment and with extreme reliability.
Benelli Super Black Eagle II with extended tube / a hamster (hamster's embody the phrase 'never quit' so they are a constant reminder to us human folks)
In this hypothetical situation where I DON'T shoot the ATF agents engaging in unconstitutional activites, I'd keep one of my 870s and all 3 of the barrels I own.