AFAIK what the military has been researching are large (naval) railguns. Coilguns are a novelty with minimal utility (note that the actual principles involved in them are used in real things, but mostly for accelerating things along rail systems, not as guns or cannons), but railguns have some promise in pushing the upper limit of what artillery can do (and AFAIK what they were focused on was specifically a hybrid system that would launch a shell with conventional propellant into a railgun barrel that would then accelerate it even harder) because they can get things moving faster than gas can expand and put more force on a projectile than conventional propellant alone could without having a building sized barrel to accommodate the force.
Ultimately the project made a big fixed emplacement that could launch a projectile faster and harder than any other system, but which was both logistically infeasible due to it rapidly destroying its own barrel (a seemingly intractable problem with railguns is that at high power levels the surface of the rails oxidizes and the rails themselves can warp), and obviated by missiles largely replacing the role of artillery along with "what if we could put a single shell somewhere near a target even faster than a missile, once, and it would take an entire ship dedicated to this task" turning out to not be as useful a niche as sci-fi brained military officials thought it would be fifty years ago.
(note that the actual principles involved in them are used in real things, but mostly for accelerating things along rail systems, not as guns or cannons)
[Legendary - Success]: Schwerer Gustav, but you don't shoot from it, you shoot it.
I truncated that down a bit because it was already getting a bit too wordy, so I skipped the examples. That would be things like some roller coasters using electromagnetic launch systems over conventional chainlift hills, aircraft catapults on carriers to get them up to speed fast enough to take off, some trains use them for propulsion, etc. It's a really good way of making a big fixed system push things along quickly for definitions of "quickly" that include accelerations humans can comfortably survive, it's just not very good at making a small and portable system for launching projectiles very fast at a speed that a human on the other end of the equation wouldn't comfortably survive.
AFAIK what the military has been researching are large (naval) railguns. Coilguns are a novelty with minimal utility (note that the actual principles involved in them are used in real things, but mostly for accelerating things along rail systems, not as guns or cannons), but railguns have some promise in pushing the upper limit of what artillery can do (and AFAIK what they were focused on was specifically a hybrid system that would launch a shell with conventional propellant into a railgun barrel that would then accelerate it even harder) because they can get things moving faster than gas can expand and put more force on a projectile than conventional propellant alone could without having a building sized barrel to accommodate the force.
Ultimately the project made a big fixed emplacement that could launch a projectile faster and harder than any other system, but which was both logistically infeasible due to it rapidly destroying its own barrel (a seemingly intractable problem with railguns is that at high power levels the surface of the rails oxidizes and the rails themselves can warp), and obviated by missiles largely replacing the role of artillery along with "what if we could put a single shell somewhere near a target even faster than a missile, once, and it would take an entire ship dedicated to this task" turning out to not be as useful a niche as sci-fi brained military officials thought it would be fifty years ago.
I truncated that down a bit because it was already getting a bit too wordy, so I skipped the examples. That would be things like some roller coasters using electromagnetic launch systems over conventional chainlift hills, aircraft catapults on carriers to get them up to speed fast enough to take off, some trains use them for propulsion, etc. It's a really good way of making a big fixed system push things along quickly for definitions of "quickly" that include accelerations humans can comfortably survive, it's just not very good at making a small and portable system for launching projectiles very fast at a speed that a human on the other end of the equation wouldn't comfortably survive.
To be fair if I'm launching the actual schwerer gustav at things I don't think "human that rides on it could survive" is high on the priority list