One of the major problems with creating something in plastic (talking hypotheticaly) to shoot a bullet is the strength of the material, friction and heat.
Strength: A gun normally uses steel. Steel has a interesting characteristic that the more pounding it gets (in this case an explosion from the round being fired) will increase its strength. Compared to cast iron the more you hit it, the more brittle. So with plastic you have to print some thing that is around 2 inches in diameter to resist a (approximate) .38 calibre from fracturing the barrel.
Friction: Each time the bullet is fired the barrel will deform from the friction of the bullet. Plastic when rubbed with a fast moving (bullet) will start to gaul. Gauling is the plastic that was in the barrel will start to stick to the bullet. A person would be lucky if the round doesnt jam on the third firing.
Heat: We use heat to melt the plastic. Yet firing the bullet has an explosion (hopefully contained). If the bullet did leave the barrrel getting the casing to be removed from the breech would be the next problem. That casing can melt into the breech rendering the print useless.

Granted this bit of theorizing is assuming the person printed in PLA or ABS. I wont speculate on what it would stake to make it better as I dont want to build a gun in plastic. I prefer a chunk of steel that I can fill with some black powder and wadding to make a bang as opposed to moving a projectile. The use of a printer to create a weapon to rob a bank is pointless, as a zip gun can be done at a local home depot for a couple of dollars in 5 minutes. Where as to print something it would take hours maybe even days (depending on complexity). A zip gun is basically a steel pipe, with a crude handle, and something that will impact the back of the casing to create ignition. Not very effective and range is very limited.

The next point is why bother? Yes some one has proved it could be done. Yet a criminal on the streets knows that there are usually a gang some where that will fence a firearm that will be far more effective than any thing printed. Private sales from some ones home is probably the easiest way for some one to get a weapon. Generally some one whom shows a polite interest in a weapon wont arouse suspicion at the time of sale. These weapons are far superior in quality and reliability than anything in plastic.