Collateral Damage

One of the biggest worries in law enforcement is having to shoot someone. And along with that an even bigger worry of collateral damage. Where does the bullet go after it strikes its intended target. Even most low powered rounds are capable of going completely through the target and striking an unintended innocent victim. And with a shotgun with buckshot you have 9-15 pellets spreading out in an ever widening pattern. That is why many departments are getting away from shotguns altogether or else switching from buckshot to slugs.
There's a type of round called "frangible". It is designed to break up on impact expending the majority of its energy immediately so as not to completely penetrate a target. Sky Marshals were the first to make extensive use of this round.

When I work in a situation where I know ahead of time there is going to be a crowd of people as at ball games or the rally, I carry the Glaser Safety Slug in my weapon. If I don't have it in my weapon I can switch it out in seconds. It is a compressed core of #12 bird shot capped with a blue plastic tip. The two main drawbacks to this ammo is the cost ($3.50/round) and a short range.