This can be done through the campaign missions. I imagine you don't HAVE to fully upgrade the railgun to get this achievement but it will make it much easier, and you get two thick shots rather than one skinny one when fully upgraded.
I acquired this on mission 27, but there are other missions that would be good for this as well. Basically, find super densely zombie populated area and fire away, hope for the best. I found the best place is on any bridge with zombies.
This achievement is also almost a guarantee in Slaughter Mode on your way to getting your vehicle fully upgraded/obtaining all gold medals in Slaughter Mode.