The sniper is planning these attacks. He would plan for the ballistic fingerprint, as well. He could easily get a stolen gun, alter the gun, or make his own barrel or other parts. You get some metal working tools and blamo, you can make a gun or gun parts.
I just thought: as soon as someone uses a home-made gun or a gun with home-made parts is used in a crime, the media and the gun grabbers will start talking about "gun-grade" metal. They'll say "what could you ever need 'gun-grade' metal for other than making a gun?" It'll never end.
Of course, low-production, hand-made, custom guns cost a lot and it will be stolen, but that will be overlooked.