The lowest that I've found in my limited research is the Barnes 70 gr TSX which expands down to 1800 FPS, which means it would be good out to 370 yards with a 16" barrel. I seem to recall Lehigh Defense claiming that their Controlled Chaos Copper bullets would work as advertised at an even lower threshold....

What's the lowest claimed expansion threshold you've seen or witnessed? Seems like that would be the true winner.