Cartridge choice is about what you are trying to stop and under what circumstances. If you are concerned with BGs jumping the fence who then have to traverse 200+ ft of open lawn to get up to a building that can be locked down in seconds and which is bullet proof, stopping power is not as important. The BG can stay on his feet for some time and still not be a true threat to the people inside the building. In this case, the armor piercing capabilities and the form factor of the firearm may make up for any diminished capabilities in "stopping power". He is probably one of the armed agents surrounding the white house that are rumored to be on the lawn in behind blinds so a bullpup is a useful tool. I have been the white house a few times, and been in the west wing each of those times (thats not the public tour, but the press briefing room and the press secretaries office) and never seen the blinds so I have no idea if agents in blinds is myth or real.