At a certain point, wouldn't the tiny fast pebble just drill through the fort and leave a small, hot hole in its wake? Like a big neutrino?
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?
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.
https://kurage.files.wordpress.com/2018/03/hocker-isbsa-14-p...
(Frankly I don't know if this makes much sense from the physics standpoint, but it's what I read in passing.)
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.
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).
https://www.radioactivity.eu.com/site/pages/Slow_Fast_Neutro...
One reason to use larger projectiles is to deliver similar amounts of energy without having to fight things like that.