Bungie had a big post about this year's ago. Basically, staggers are applied in stacks, and each weapon archetype applies different amounts of stacks, but rapid-fire weapons apply it faster, and high caliber rounds further increases the stacks, reducing the time before flinch. As I recall in that post they mentioned that rapid fire pulses was the best weapon for HCRs and both hand cannons and scouts were the worst due to their slow ROF.