Why would you want your projectile stuck in orbit? What you’re shooting at is on the ground.
You wouldn’t. Reread the exchange.
This also isn’t an instant shot/kill (meaning ‘bang you’re dead’).
At those speeds, the shell is moving about 1.5 miles per second.
Even if it didn’t slow, time on target at 100 miles would be about 67 seconds.
In that time, a ship moving at 30 knots will have moved over a half mile.
A jet moving 600mph will have traveled over 11 miles.
But since the shell does slow, the numbers are even larger.
You still have to time it and aim where you think the target will be.
A projectile orbiting 10 feet above sea level will do a number on any ship taller than 10 feet that gets in the way. 
The reality of course is at 10 foot altitude the “orbit” will decay pretty quickly due to air drag. So the projectile may only travel a few dozen miles before it slows down & falls into the sea.
You wouldn’t fire a naval railgun at other ships unless you had to. Missiles have greater range still, and you’d try to kill the enemy ships with a volley of missiles fired from outside the enemy’s range if you can. And you’d try to use airplanes over missiles.
The documents I’ve read on it basically say it is a cheaper weapon per ton of firepower delivered than a missile. So it’s supposed to be used for long range naval bombardment of targets on land mainly. The shells would have to be guided, and the 100+ mile range would make a far larger portion of the world within range.
Or yes, as an air defense weapon, since it would have a longer reach than anything slower.
What would be the ideal landbound target other than a weapons depot? The projectile is going to pass through any building like it was a cardboard box. Sure, the first room will get brick or concrete shrapnel, but spalled sheetrock isn’t going to be a serious hazard problem in the remaining rooms before the slug passes through the outer wall.
Maybe the accuracy will be good enough to take out individual tanks/planes but those seem like a better target for conventional explosive warheads with a larger kill zone.
It’d start fires, which is what kills ships.
On a smaller, slower scale, this is how tanks kill other tanks; modern tanks fire solid sabot rounds that punch through armor. The kinetic energy transferred from the round to the tank’s hull is in part turned into heat, and the inside of the tank is filled with fire and molten metal. It’s effective, to say the least.
A hyperfast round striking a ship would have the same effect; as it hits the hull, and its component parts hit other parts of the inside of the ship, the result is enormous amounts of heat. Fires start and stuff burns and spreads all over the place.
When HMS Sheffield was struck by an Exocet missile in the Falklands War, what destroyed the ship was not that the Exocet blew it up or put a hole in it that the water rushed through; what destroyed the ship was that the missile’s warhead and unspent fuel started fires the crew could not get under control, and so they had to abandon the ship. A hyperspeed round would have the same effect, simply exchanging the warhead and fuel for sheer kinetic energy.
For buildings, I would assume to goal is to impact the ground in the middle of the building, giving it a mini-GodRod effect. Railgun speeds are somewhat less than kinetic bombardments and weigh significantly less, but come with the bonus that you don’t have to lug literal tons of useless metal in to orbit.
You’re presuming it’s going horizontal at extreme range - it won’t be. Then there’s thermal shock, HUGE shockwaves from atmosphere moving about - Lethal overpressures. There there is the thermal component, which sets things like gear and clothing on fire… Then there’s all the fun from the various bits of infrastructure bouncing around. It’s not going to be anything other than hellish to be on the recieving end.
Basically, anything that can’t move out of the way in about a minute is fair game - Stationary vehicles, or vehicles on a predictable route. Bridges. Bunkers. Trenches and foxholes. Buildings full of forted-up fighters. Anything that can serve as a strong point is a target… They force the enemy forces into the open, where they can be easily defeated.
But there is lots and lots and lots of ship compared to just a teeny tiny bit of tank. The crew of a ship can always come over and put the fires out with fire extinguishers and pump the waters out, plus a ship can have lots of holes put into it and still function quite well, whereas a single tiny hole could completely knock a tank out. I mean, how many railgun bullets would be required to put a ship down if they just put a hole through it and start some fires? And what if it just hits empty metal hull, or a place where the ocean would quickly put the fire out?
Sheetrock isn’t that common of building material in most parts of the world. Besides, a railgun is only one of several options.
Heh. You’ve never done Naval Damage Control, I can tell. 
Start off - Remember I used the phrase “Mission Kill” way up above? That means “Incapable of carrying out mission - might as well go home.” Now, remeber the USS Stark? Not sunk, but mission incapable. The USS Princeton - Mission Incapable. USS Cole? Mission Kill. You don’t have to kill a ship to render it meaningless in terms of combat.
Second of all, these are WARSHIPS we’re talking about - they’re FULL of things which burn at very nasty temperatures. They’re made of metal, which conducts heat - and full of ventilations, power, and data runs which allow fire to sneak past and behind the crew. Any ship will burn. Warships burn HOT.
Yes, we’re trained to deal with it*****; but whilst you’re putting out the fire, you’re not fighting the enemy (except in as much as fire is every sailor’s enemy!). One compartment? Yeah, we do that with ease. Two? Not as easy, but OK. Three? Now you’ve got a problem. Four? Oops - you are most likely OUT of the fight. Five? You’re in a fight for your life.
Those HV Projectiles? They’re going to trash systems and spaces, and start a LOT of fires in multiple spaces.
*****Been there, done that. Multiple times. It sucks - It’s frightening. No choice but to deal - you can’t walk home.
Understood, but the point was that interior walls aren’t usually made of something that’s going to be a threat to everyone if a mach+ projectile comes through it. Sure someone might catch a 12" splinter somewhere serious, but you can’t count on it happening.
As a bare idea of what a fire shipboard looks like, here’s some Naval Damage Control Firefighting training. The real thing can be FAR worse.
Edit:
Those spaces are VERY clean, with very few of the actual systems & fuels present in a real warship.
Or a bunch of bricks or rocks… But the overpressure alone will screw 'em up.
Still, with a tank, you’re probably (?) more likely to hit something critical, surely. There’s the crew compartment, hit that and the tank is absolutely useless. There’s the engines, hit that and at the very least the tank doesn’t go anywhere. There’s the ammunition, in any case the tank will be usually useless. There’s the actual guns themselves, they will be destroyed, there’s the treads and spinning bits, the fuel tanks and all the other stuff all crammed into a small vehicle, but with a destroyer, there is so much empty space and redundant systems.
Also, aren’t most critical areas usually armoured?
But anyway, with a tank, the shot almost HAS to hit something important, but with a ship it might just hit some empty hull, some crew spaces, some more empty hull with nothing connected to it. And don’t warships carry a large crew? What about the janitors, chefs, entertainment ladies, clowns, PR staff that tell civilians that the USN are not supposed to be mindless murder machines and mechanics? For at least a small amount of time, they could probably pitch in and suppress the fires while the important crew defend themselves?
You’ve never been inside a destroyer, have you? Ships are LARGE systems - with large support requirements. Sure, it’s not as cramped as a tank - most places, anyway. But those systems and spaces are LOADED with critacal electrical, data & electronic, and various hydraulic and pnuematic systems Pipes carrying everthing from pressureized air to firefighting water to highly flamable lube & fuel oil… Distribution panels & switchgear. Generators. Radar. Sonar. Berthing (which burns like a salamander on crack!)
Nope. Too much weight. Ships are mostly defended by 1) Not being hit, and 2) Damage control. Sure, some small areas are armored against fragmentation (or to protect the grew is something disasterous goes wrong inside), but if you get a fire in your main switchgear, it doesn’t matter anyway - your oh-so-important Inertial Navigation Binacle is useless until power is restored.
You’d be surprised. During Desert storm, I saw a Bradley fighting vehicle take a direct hit from an RPG, and carry on - after greasing the poor brave bastard who shot it. Tank creas have had panetrations which killed or wonded crew but didn’t findamentally disable the vehicle. But those are rareties. Likewise, a warship doesn’t have alot of ‘non-critical space’ - everything that it is there, is there for a reason. Sure, you can poke a lot of holes without doing real damage - so long as you avoid fire. Once you set things burning, all bets are off.
Heh. Of coure they do. Assuming of course they didn’t suck up a bunch of fragmentation themselves. Read up on the Stark or the Cole, and see how hard they had to fight to stay alive. Because There are very few non-essential personnel on a warship - when you go to battlesations, everyone has a job. Including damage control. Once you go past a certain amount of crisis, though… Well, so much for doing combat. At that point, you’re just a target bobbing along.
Navy warships are not, contrary to popular belief, pleasure ships. If you can do without, they probably already have.
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Yeah, ships are not mostly hollow, where something like this just punches a clean whole and people barely notice it.
That first shot was one Wile E Coyote would definitely appreciate, shooting what is basically an anvil at mach 5.
It reminds me of the efp that the bad guys used in Iraq. It would punch a hole through basically any armor and catch everything inside our vehicle on fire.
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