I have to wonder if on their night shift at Walmart, part of their plan was to steal some ammo for their plan to do what they did.
It could have been their plan but very difficult to do. Certain, dangerous or high theft items come off the truck and go directly to a locked cage in the back, under manager supervision.
They wouldn't have been employed nearly long enough to get anywhere near them and there are more cameras in the back than there are in the front and ammo comes into the store alarmed so even if every other safety procedure in place failed, they would have set off alarms leaving the building.