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.