I don't no much about weapons but if he had used that kind of ammo, wouldn't the gun be kinda useless against zombies? I don't think anyone would use it, because it is much more likely to run into a zombie than having to take out several living targets at the same time.
But this kid wasn't out to shoot walkers. He was part of a group sent to kill the group of living people in the prison.
A shotgun loaded with the proper ammo, can do a lot of damage. Never under-estimate it.
Exactly and it's the perfect weapon for an inexperienced shooter against the living OR dead as you pretty much have to point in the general direction and pull the trigger.