It's not about legitimate purposes, and even if it were, who the hell does the non-elected public employee, Chief Bratton, think he is to determine what is, and what is not, "legitimate"? "Shall not be infringed", doesn't say anthing about "legitimate", for which they really mean "legitimate sporting", purposes.
Actually a .50 BMG rifle would have come in right handy when those armor wearing bank robbers were spraying bullets hither and yon in the Chief's jurisdiction a few years ago.
That's the truth! Still the whole thing could've been ended if any of those 50 cops had been a good enough marksman to score a head shot with a little ol 9mm. Heck, when they hit the nearby gunshops for backup firepower to make things "equal" not realizing that volume of fire without hits does not "firepower" make...they should have taken a deer rifle like maybe even a .30-30 and gone for the head shot then. Sheeesh I think that whole situation was a point of national shame for the LAPD for their lousy marksmanship.