Well now we're back where we started. Compare a theoretical Metalstorm handgun against an existing mechanical handgun.
...a big clip that fed all of the barrels new bullets + propellent through the back via some sort of rotating device.
The mech handgun has about 16 bullets in its clip, fired as fast as the trigger can be fired, with chance of jamming, and excess shells ejected.
The Metalstorm handgun can have at least three barrels in the same space (though with the size of projectiles used today, I'd guess at least 8), with at least 6 shots in each barrel. I believe that this is a very modest estimate. Either way, assuming the same firing system of as-fast-as-you-can-fire, and electronics allowing barrel linking, you could fire the same number of bullets in 3/16 the time. But that hardly matters. What matters is that each barrel could contain a different type of projectile, meaning that the cop does not have to shred a suspect with a barrage of lead, but rather can switch to a non-lethal deterrant ammunition, such as rubber bullets, without carrying or drawing an extra piece of equipment. This allows faster adjustments during a mercurial tactical situation. This is good, and could work to reduce injuries in such situations.
Replacing a horizontal clip is probably no harder than replacing a vertical clip. Honestly, guys, think about it; They'll work out an ergonomic system for Metalstorm systems, too. You won't be hiding in a corner, reloading a metalstorm gun with a ramrod, a barrel of gunpowder, and a little baggy of metal balls. That's a flintlock.
I reiterate a belief I've held for years: Explosive triggering systems are easier to work with than air-actuated. I'm sure that's why we had them first.