Slam-fire shotgun can be built, in a day and in total secrecy, by a 12 year old with some loose change in any modestly equipped garage. It is extremely easy to conceal and carry, you can reach 30+ shots per minute with little training and, sadly, ammunition for it is probably less time (read: resource) consuming to acquire in many countries than the knowledge of electrical engineering sufficient to build a working railgun of that scale.
Never heard of slam-fire shotguns before. I'll look into it. As a kid I had seen designs for railguns though.
I disagree that the shotgun would be easier because of ammo issue. It may be as you say in the US of A, but in Europe it's not that easy to get your hands on live rounds. Also, I doubt people often sell ammo to a 12 year old - so in order to get it, you'd have to interact with a grownup. Railguns are easier; your parents will never know what you're building even if you're spending your lunch money on high voltage capacitors.
