The CZ550 is pretty common. I've come across it numerous times now. The other snipers are a bit rarer. I've checked all airfields and most other military buildings numerous times and so far I havent come across any sniper. It might just be bad luck, but it can get quite tedious. But hey, who said a zombie apocalypse would be a series of events devoid of dissapointment? That's right, no one.