Assuming power can be provided, what would be the more practical primary armament for a tank using an electromagnetic cannon? I am looking for a good compromise between accuracy, cost and durability of the cannon. Assume that high temperature superconductors are common place in this universe.
Coilguns (assuming current technology)
Ironically, rail guns have arguably received more attention for development in military applications. Unfortunately, however, there have been some major setbacks. The Navy is documented of having been developing rail guns as weapons as far back as 2005, yet as of 2018 there have been some hugely limiting problems:
There are a number of things the Navy still doesn't know how to do. How is it possible to shoot more than about three or four rounds out of an EM rail gun before having to change the barrel? How is it possible to build the kind of pulse forming networks that are needed to be able to shoot not direct fire weapons, but 200 mile indirect fire weapons? There is a lot of potential that a capability like EM rail gun could bring. But there are many risk to EM rail gun.
Railguns face two types of problems : engineering problems, and physics problems. Engineering problems, such as the size of pulse power units, are engineering problems that can be resolved with suitable applications of time and money. Physics problems, such as barrel life, are rather less amen able to such brute force solutions, and are the reason that not everything that can be described can be built, or built in a useful form.
Wikipedia summarizes the challenges of rail guns (emphasis mine):
The heat generated from the propulsion of the object is enough to erode the rails rapidly. Under high-use conditions, current railguns would require frequent replacement of the rails, or to use a heat-resistant material that would be conductive enough to produce the same effect. At this time it is generally acknowledged that it will take major breakthroughs in materials science and related disciplines to produce high-powered railguns capable of firing more than a few shots from a single set of rails. The barrel must withstand these conditions for up to several rounds per minute for thousands of shots without failure or significant degradation. These parameters are well beyond the state of the art in materials science.
This really only leaves coilguns - mostly for their capacity for fire regularly. Some important items of note:
- Although coilguns typically have lower velocities than rail guns, tanks don't need to fire at the same distance/velocity as the Navy needed in the first link above
- The biggest issue with coilguns is timing, but the issues with timing in coilguns dates from WWII on to the 1980s, which is now much easier to deal with via computers.
- NASA has many uses for coilguns, and is actively pursuing it's usage
In conclusion, although the railgun has arguably received more attention, there are some strong material limitations with it's usage in war. Coilguns, on the other hand, are limited mostly by timing and design issues, which are much easier to deal with today, and therefore make coilguns more practical for tank cannons (assuming current technology).
Railguns. Because you are writing awesome fiction, not working for DARPA.
Coilguns have a limited upside for awesomeness. At the end of the day they are glorified doorbells. Yes they are more practical. So is commuting with your Prius as opposed to your jetpack. If you are trying to shoot things in real life, work on the coil guns and you may be on the wrong website.
Railguns can be made into sweet near future scifi with nonmagical real physics-based schemes. You know that because already are thinking along those lines with your high temp superconductors.
Using a superconductor for projectile or rails sidesteps much of the current issues with ohmic heating and rail ablation.
Another idea I had for railguns was more plasma.
Plasma armatures are used on some railgun projectiles - the armature is behind the projectile and gets turned to plasma when the electricity comes on. The plasma then conducts the electricity, closing the loop that provides propulsive force. Leaving the projectile out of this means more options for projectile composition. Why not do that with the rails too? Regenerate your plasma rails after each shot! In my scheme, graphite is used. I like graphite also because if the condensing graphite plasma were not vented appropriately, once they were in a real fight your tank crew would soon look like chimney sweeps.