>>13909980Because it raises complexity on an already delicate piece of machinery over just using fins to stabilize the projectile. The reason guns don't fire bullets with fins (usually) is because you are blowing shit up inside of it which basically translates to it having to be round to avoid weak points and thus fin designs would have to be sub caliber (or require a bigger gun) and even then most tanks for instance choose the smoothbore these days because it allows options in the ammo you load. A railgun doesn't share similar limitations since the thrust is provided by electricity the "barrel" has much more freedom in terms of size and shape than a gun so you can easily mount projectiles with fins. And of course because a smooth bullet and rifled barrel are lot cheaper to manufacture than finned bullet and smooth barrel, while with a railgun the dynamic flips where straight rails are cheaper than curved ones by wider margin that the savings a smooth projectile has over complex one.
Also the US navy railgun does spin it's projectiles, you can see it pretty clearly on the various videos with the projectile with the fins.
