Zombies gather in groups when they hear noises that attract them. It's not a grouping behavior per se, but rather that the individual zombie reacts to the same stimulus and moves toward it, so it is natural that they will bunch up when the same stimulus reaches a bunch of zombies at the same time. When you observe a group of zombies, absent any stimulus, they will just mill about, bumping into each other, and eventually scatter. Shotguns can blow a head off a zombie, but you need to close and if you are faced with a lot of zombies, the limited capacity, slow reloadability, and heavier weight of a shotgun can really be more of a hindrance and put you in danger. Robots without specific programming can wander around, but if programmed with a mission, like the Terminator, e.g. T101, T1000, or TX, they will be hard to pick out as such and a sniper rifle will be of little use against the T1000 and TX. Even a T101 can withstand quite a bit of 7.62Nato fire. Suppressors are a great idea with zombies so they don't hear the gunshot and congregate on your location. Robots made after the clone wars have very sensitive acoustic receivers and can register gunshots from miles away and pinpoint the caliber and your exact location. Zombies have no emotions. They cannot hate, love, distrust, etc. They are dead bodies animated by Solanum with only the desire to eat flesh. At 400 yards, a 7.62Nato round will not carry enough energy to penetrate most exoskeletons of post-Clone war robots. Zombies do not possess the locomotive ability to jump. They usually shamble along and sometimes crawl if they have already decomposed or suffered damage to their legs. You are thinking of the "infected humans" seen in "28 Days Later" or "I am Legend". Those are not zombies per se, they are still humans who have been infected with a virus that does not kill them, but rather, instilled a hunger or rage in them to feed and kill humans. They are still living and breathing, which is the key difference between them and true zombies, who do not need air as they are dead. But fear the man with lots of guns and lots of ammo too!