You fire the weapon. The tiny pebble breaks up on impact, creating a little crater in the rock. To you, the medieval siege engineer, this is annoying. The frenchmen taunt you from their parapet, yelling maternal insults and farting in your direction. Your supersonic trebuchet is a failure, tactically speaking.
You can't take their mockery anymore, so you set about fixing your projectile.
The pebble lost much of its energy in flight - drag is a huge issue with any distance to target. You make a projectile in the shape of a rod. This has the effect of decreasing the cross-sectional area of the projectile relative to its mass, minimizing drag losses. There's a limit to the length:width ratio, however- go too skinny and it'll deform under firing forces.
The projectile's length also allows it to burrow deeper into the rock -- when material at the tip breaks off on impact, it's got the rest of the shaft behind it, just as mad.
To pack the most punch into your projectile, you craft it from tungsten (then known as Wolfram). In order to keep it from yawing in flight, you add some rigid vanes at the back. It looks like a small, superheavy arrow.
Now you have the issue wherein it doesn't fit well into your cannon anymore. No mind -- you set it in a cup of sorts that seals the barrel. Wait, where did you get a cannon? This is the year 1217 AD...???
Congratulations: you have constructed an armor-piercing fin-stabilized discarding sabot munition (APFSDS). You load it into your M256A1 120mm smoothbore cannon. You peer through the sights.... now where were those sac à merde?
This would be a highly unexpected turn of events in 'The Holy Grail', at the same time there were already some temporal discontinuities so in a way it fits right in.
Weirdly, that's the idea behind naval guns from the age of sail.
Solid shot goes through the ship, perhaps injuring anyone in the way. Solid shot hitting the structure of the ship on its way through creates massive, dangerous splinters.
This happens with armor piercing shells hitting tanks or other armored vehicles, too. And even if they don't penetrate, they can still generate spall from the inside of the armor that's still deadly. (This was at least true with ~WW2 tanks, modern armor is more advanced.)
Afaik that's exactly what happened with railguns: they were tested at something like mach 11, but it turned out that they just made nice holes in the targets, instead of delivering the energy to the targets themselves. So the speed had to be dialed back.
(Frankly I don't know if this makes much sense from the physics standpoint, but it's what I read in passing.)
This is something that's hard to appreciate with any energy-transfer mechanism: If you reach energies that exceed the capacity to transfer energy, you're just wasting effort.
For kinetic systems, the projectile-target interaction is critical. A flimsier target actually needs an impact to be spread over a larger area. Think of a pellet shot through tissue as opposed to, say, a blast of sand or salt.
There are some analogues with subatomic particles and fast/slow neutrons (slow neutrons are far more effective at sustaining nuclear reactions, as they effectively "stick around" where they're more likely to interact with other nuclei for a longer period of time, and the apparent cross-section of the target nuclei appear larger).
The railgun test videos I've seen were a lot more than "just make nice holes."
At that energy level you have significant damage from spalling from anything the projectile passes through, the superheated metal the projectile actually passes through is launched everywhere, there's the shockwave of the projectile, etc.
That's easily enough to be anti-personnel, incendiary, and obliterate electronics and cables and pipes and hoses. And have lots and lots of penetration, which on a ballistic arc could mean exiting below the waterline, and I imagine something traveling at supersonic speeds hitting water probably does some pretty impressive things.
If you send one of those projectiles through a magazine, missile battery, or fuel bunker - a lot of people are going to have a very, very bad day.
I'm a little doubtful. I think your standard pebble would vaporize from the friction with the air. A pebble certainly wouldn't survive re-entry into the atmosphere, so there's an upper bound on the speed before it disintegrates. If you made a vacuum between you and the wall, it might work?
One reason to use larger projectiles is to deliver similar amounts of energy without having to fight things like that.
A small projectile like a pebble would be vaporized pretty quickly by friction with the air at those speeds (and of course a bigger projectile would take more energy to launch).
