It's generally thought that shotguns are a better bet than either bolt-action rifles. The trade-off with both the shotguns and the semi-automatic rifles is that although they're loud, their ability to put down waves of zombies is so impressive that sometimes it's worth it. That isn't as much the case with a bolt-action rifle: you won't get the extra stopping power benefits, but you will get a big group of zombies alerted to your presence.