"Spraying" bullets is effective in two places. First, to lay down a cover by a group of men. Second, close-range shooting. Close-range shooting is better with a sidearm anyway.

You simply can't expect to have good accuracy on the second or subsequent shot of any hand-held machinegun. They're best when you just don't care who you hit.

I don't recall many news reports of crimes committed with machineguns in the US, other than unregistered possession. They really aren't a problem. Perhaps it's the fact that they've been controlled for a very long time, but perhaps there's not a real advantage in the real world. I vote for the second; criminals can get whatever they want.

There's a kit on the market to convert some Glock pistols into machineguns. It consists of a small insert to replace the rear cover on the slide - there are pictures on the BATFE's website. It's a scary thought that people are running around with these things, but I'm sure they are. And they're not the law-abiding sort.