The S12 Shotgun is fairly effective against the zombies, with 10 shells in a magazine, and being semi-automatic. It is a 1 hit kill until round 12, ironically enough. While mostly disliked by players who enjoy shotguns like the Remington 870[my personal favorite] and the KSG, the S12 serves a much different role. The other BOll shotguns are great for training, when your running around and someone gets in your way, with a Remington 870 or KSG it is easy to clear them with one or two shells instead of attempting to dodge around them or change direction, people that try to use the S12 like this are disappointed and often trade it for a different shotgun or different gun altogether but if you use the S12 like you would an assault rifle or SMG and shoot into the hoard its high ROF and healthy clip size will make up for its lower damage and can kill a massive chunk of a train with one magazine effectively up to around round 22-23. The stronger damage slower ROF pump shotguns are impractical for this use because it simply takes too long. Most shotguns in BOll have a fairly small pellet spread but the S12s shells seem to spread like butter making it even better for effecting multiple zombies at once. While the pump shotguns have great DPR[damage per round], the S12 has much larger DPS[damage per second], this means each gun can't do each others job, S12 for killing trains, pump shotguns for building them up. Switching the two rolls up would be like trying to spread jam on toast with an ax and trying to cut down a tree with a butter knife, keep that in mind when using the S12. Despite the amazing crowd control capabilities the S12 carries only 40 rounds in total so it isn't something you would want to stick with forever but if you get this out of the box on round 20 its worth holding onto and is capable of finishing that round if you group up your train effectively and don't waste ammo. Overall not as bad as some make it out to be.
Upon pack a punching the S12, you will get the Sythetic Dozen, still semi-auto, still 10 shells per mag, but with 10 more outside than before.