The Russians only had unarmed security at this venue. What they did have were plenty of walkthrough metal detectors, who stood watch over the carnage. Most concert venues around here at places like the Performing Arts Complex or Fillmore Auditorium on Colfax similarly have mechanisms to fully disarm patrons, but only have minimal armed security or LE presence on-site. The one scenario that is completely different are sporting events- plenty of fully-kitted LE at Broncos/Nuggets/Avalanche/Rockies or Buffs games that keep things under control on the inside, but still leave the crowds waiting outside the checkpoints pretty exposed.