I think the risk-to-reward ratio is pretty low for having an illegal automatic weapon. It takes a lot of trigger discipline even for average trained soldiers to not to quickly waste most of their ammo with fully automatic weapons.

Then again, just because it's not smart doesn't mean people don't do it.

It really depends on what their objectives are; what they think they might accomplish by having an automatic rifle. There are a wide array of tactical objectives, some better served by automatic weapons than others. Consider particularly the different tactical objectives terrorists and soldiers might have. Particularly, soldiers need more discipline because they're likely to be fighting people who can fight back. Having bullets come back at you would surely fray the nerves.

Point taken, but I think most people tempted to have illegal automatic weapons are the paranoid home defense crowd, not the massacring unarmed civilians crowd. They're probably imagining having to use them against SWAT and military.

Err... I mean reward to risk ratio is pretty low... probably even negative advantage gained, and lots of risk exposure.