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Alright, this will take some time to explain. There is historical context to my question.

Over the course of the period of time from 16th century to the first half of 19th century, the European muzzleloading naval cannons were progressing towards more muzzle velocity(up to 500m/s), more accuracy and heavier projectiles. While there were some exceptions in this development, such as carronades, excluding those unusual developments, the overall cannon qualities were obviously improving greatly. Optimal length of a cannon(which imparted most of the kinetic energy into cannonball) was found - so called "long guns", the boring of cannons allowed smaller windage etc. Eventually, shells replaced the cannonballs, but before that, a lot of progress was made and the difference between the 16th century 32 pounder demicannon and the 19th century 32 pounder long gun was massive.

However, during the 16th century, there also existed completely different style of naval cannon: in Korea. Called Hwapo, Hwatong and later Chongtong, these cannons could, just like their European counterparts, fire regular cannonballs, but also, more interestingly, could fire a bolt of a shape that somewhat resembles modern missiles.

Chongtongs

According to partially confirmed information, largest of such cannons "cheonja-chongtong" which fired 30 kg (66 pounds) heavy bolt had a maximum range of ~ 1600 m. With help of some math, I eventually ended up estimating its muzzle velocity to be around 140-170 m/s. These cannons were being improved from 15th to 18th century but except for minor improvement, there was no major overhaul.

In fact, the bolt fining cannons were gradually pushed out by Chinese take on European Culverins named Hongyipao.

Muzzle velocity of 140-170 m/s that I calculated is greatly below what the potential of black powder cannon can do, so it appears there is lot of space to improve that. However, that would require a thicker cannon, that can withstand a larger charge (Cheonja Chongtong apparently used only a bit over 1kg charge to fire the 30 kg bolt) and also making the cannon longer would improve its efficiency. Actually without making the cannon longer, the improvement we can achieve is limited.

Which is where the problem is. These bolts have to have fins to have stable flight and accuracy. And so, making the cannon longer without making the bolt longer poses a problem, as in the historical version, bolt was put into cannon in a way that fins were in front of the muzzle.

In a fiction I am writing, it was my intention to have these cannons be developed with natural progression towards something like modern sabot ammunition, where the problem disappears as you're now able to fit the whole bolt into the cannon, with not even tip poking out, allowing the cannon to be as long as needed. Need for this development is justified by the existence of magically hardened wood used in naval ships.

modern sabot ammunition

However, I am having doubts about realism of developing sabot ammunition with what is basically 17th/18th century metalworking. I'm thinking that there might perhaps be simpler method to reach that that I overlooked. So, could bolt firing black powder cannons be realistically improved to have their bolts reach high (400m/s +) velocities, or is this beyond the means of the era I set this in? Or are Korean bolt firing cannons a dead end, and bolt shaped cannon ammunition is just outright suboptimal before technology such as smokeless powder come into play?

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    $\begingroup$ Oooh, I never knew these existed! Neat! $\endgroup$ – Andon Jan 11 at 16:36
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    $\begingroup$ Accuracy was not a very important quality in naval cannon in the age of sail; the problem being that it is pretty much impossible to aim accurately a black powder cannon at sea -- the firing ship is moving on three axes, the target is moving, the propellant has inherently unreliable caracteristics, the delay between yanking the firing lanyard and the actual fire is unpredictable. What counted was the total weight of a broadside and the rate of fire. Wind powered men'o'war engaged the enemy very very close, under 100 meters; firing from a mile away was seen a waste of powder and cannon balls. $\endgroup$ – AlexP Jan 11 at 16:44
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    $\begingroup$ All I read was ‘Korea’ and ‘Navy’. I hereby invite you to learn all about the life of Admiral Yi the one true Marshal Lord of Loyalty! Read! Explore! And discover how he probably thought of a solution to both executing and defeating your idea in combat! Haha! $\endgroup$ – Darius Arcturus Jan 11 at 17:19
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    $\begingroup$ Sorry, I love history, haha. $\endgroup$ – Darius Arcturus Jan 11 at 17:20
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    $\begingroup$ Thank you for the history lesson. Having made firearm sabots out of plastic, I am certain you could make them from wood or other materials available in the age. Creative progression is something I would enjoy seeing. Who "invents" the idea of fitting the fins in the tube or making the rod sub-caliber? $\endgroup$ – UrQuan3 Jan 16 at 17:03
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Here’s one option that might work. When the charge (green) is fired the main projectile (red) is blasted down the barrel of the gun (grey). The fin section (blue) is a simple hollow cylinder with fins attached of roughly the same bore as the central projectile spindle. When the charge is fired this section is hit by the back of the projectile as it leaves the cannon and sticks there jammed against some wood, wadding or piece of lead. The longer extensions to the gun are for guiding the fins and might not be needed.

enter image description here

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  • $\begingroup$ I'll have to look more into it, but it's a damn ingenious idea. $\endgroup$ – Failus Maximus Jan 12 at 17:42
  • $\begingroup$ The Problem I see is here is, that the fins might break of if they hit the back of the bolt (or rather are hit by the back of the bolt) $\endgroup$ – Sebastian Jan 12 at 20:28
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    $\begingroup$ Just need to make everything strong enough. The force on the fin section would also be reduced by the use of the wadding or wedge spreading the acceleration force over a fraction of a second as it compressed. $\endgroup$ – Slarty Jan 12 at 21:49
  • $\begingroup$ I love the idea. Sebastian is correct, but there might be a good solution. $\endgroup$ – UrQuan3 Jan 16 at 17:07
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Modern sabot rounds work for a very specific reason -- at the extreme velocities they strike, the hydrodynamic penetration process (the armor and projectile both erode away) means that a very high length/diameter ratio is more effective. This would not really apply to your scenario.

However -- they key thing will be the failure mode of magic wood. Is it brittle? Does it bend and cave in? Does it crack, or break into large/small/dust-size pieces. This will determine the best sort of projectile.

That said, a sabot projectile - which you can do perfectly well with black powder tech - is still a good way to concentrate maximum force on minimum area, so it this is what is needed the break the spell, go for it.

Archaelogists looking at the wreck of the Mary Rose have found what they claim are early amour=piercing rounds: "Powerful imaging technology has revealed cubic-shaped lumps of iron encased in the soft, lead cannonballs, which would have allowed guns to punch through the sides of enemy vessels. … The cannonballs would have worked much like a modern-day armour-piercing round — the soft outer material, in this case lead, would have deformed on impact, throwing the hard iron core through the armour plating. The trust hopes to conduct tests at the The Royal Armouries to try to better understand the damage they would have inflicted. Alex Hildred, of the archaeology team, said: “We first noticed something strange when the lead cannonballs began to rust. Some burst open so that rust came almost pouring out of where it had split. “We found there were small iron cores inside. This could have been done to make the cannonballs cheaper or it may have made it easier to make while at sea if you had an iron core — you had to carry less of the heavier lead. “But they would probably have worked like a soft-nose bullet. They could be some of the earliest examples of armour-piercing projectiles."

You could use your sabot projectiles similarly to deliver maximum force on a small area to splinter their way through..

Aerodynamics might be challenging -- but on the other hand, if they get it right, a 'winged' projectile might even have greater range and accuracy than an equivalent cannonball. Unlike other projectiles, sabot are often finned rather than relying on spin-stabilisation, so gliding wings might be viable.

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  • $\begingroup$ The wood still breaks into splinters, only it is harder to break and penetrate it. $\endgroup$ – Failus Maximus Jan 11 at 21:31
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I'm not certain why you're rejecting sabot rounds. A sabot is merely a disposable piece of hardware meant to position the round correctly and seal in expanding gasses, thus maximizing the amount of kinetic energy transferred from the charge to the round. If you increase the size of the barrel to fit the entire bolt, then fit the bolt with two thick rings of (say) bamboo, with the lower one faced with iron to keep it from cracking or burning, that should suffice as a low tech but effective sabot. Just make sure each circle is split so it falls away immediately after firing.

You could also consider the possibility of spring-loaded fins, which would allow the un-saboted bolt to fit entirely in the barrel, with the fins expanding as soon as the bolt takes off. That was certainly within the technology of the day, though it adds an inconveniently breakable moving part.

I don't know how effective these bolts were in general. I imagine a cannon ball was a more effective anti-ship weapon for the period, since it produced a large-area crushing force as opposed to a pointed piercing force. But I also suspect that part of the problem was standardization. A large-bore cannon meant for these saboted bolts would be useless for cannonballs, requiring two separate cannon for the differing ammo. Part of the reason that modern sabot rounds have become established is that they allow smaller rounds to fit within common standardized barrels, and the piercing design fills a modern need for penetrating heavy armor with explosive gasses. Bolt-type ammo might make sense for penetrating your magically-hardened wood, but only if it delivers some secondary effect after penetration: an explosion (explosive rounds are a bit advanced for 18th century technology, but not impossible in a crude way), a magical effect, or anything that would damage or disrupt the ship or the crew from the inside.

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  • $\begingroup$ "anything that would damage or disrupt the ship or the crew from the inside." like, say, being hit by a heavy chunk of high-speed metal? $\endgroup$ – Starfish Prime Jan 11 at 19:15
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    $\begingroup$ Actually Korean cannons could fire both cannonballs and bolts. Better yet, they could also fire grapeshot. However, for reasons unknown to me, generally, they prefered bolts over cannonballs because of accuracy at long range reasons. This somewhat confuses me, as it kinda goes against Western naval tradition where accuracy started being really important only by 19th century. Though, now that I think of it, I know the reason. Their enemy was Japanese, who used musket sweeps and boarding with nearly no naval cannons, so prefering accurate bolts makes sense. $\endgroup$ – Failus Maximus Jan 11 at 19:23
  • $\begingroup$ @StarfishPrime — Cannon balls/bolts are not really effective as anti-personnel weapons. I mean sure, a 22 pound cannon ball will kill a person, but so will a one ounce bullet. Cannon balls are meant to break up hardened structures: to knock down masts, to damage equipment and decks, to blow holes in the sides of a ship so they sink. If you want to use cannon against people, load it with grape shot or chain, and use a shorter barrel so you get a scattering effect. If you want to use cannon against a ship, use long barrels for distance and massive rounds for destructive power $\endgroup$ – Ted Wrigley Jan 11 at 19:23
  • $\begingroup$ @FailusMaximus — yes, I was aware. I'm just saying that to set up the bolts as a sabot you'd need to expand the barrel of the cannon or drastically reduce the width of the bolt to make it compatible with a standardized barrel $\endgroup$ – Ted Wrigley Jan 11 at 19:26
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    $\begingroup$ @TedWrigley: Actually, in the age of wind-powered wooden ships, cannon balls were highly effective anti-personnel weapons. The thing is, wood splinters. When a cannon ball hit the wooden ship, the wood would splinter into hundreds of sharp pieces which were thrown off at high speed, wounding or killing anybody in their path. $\endgroup$ – AlexP Jan 11 at 20:27
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Looking at the design of the round, it seems that what is really desired is a form of "Spigot Mortar" rather than a cannon.

enter image description here

WWII era Spigot Mortar

The illustration shows the gun crew aiming the small, lightweight spigot (essentially a long steel rod) with the mortar shell already placed on the spigot. The propelling charge is actually in the cylindrical tube attached to the warhead, and after firing, the tube falls away exposing a set fo fins.

While a WWII era spigot mortar is designed to create a small, inexpensive and lightweight weapon, there is no reason that it cannot be scaled to larger sizes. It also provides the option for the gun crew to choose different sorts of ammunition rather than be confined to metal "shot", they could have explosive Shrapnel style shells to clear the enemy decks, or canister shot for very short ranges to repel borders.

Having an arrow shaped "dart" isn't much of an issue with a spigot mortar, and with the short ranges that naval combat took place in during the age of sail, the weapon should be relatively effective.

The main downside of spigot mortars is they exchange the weight and expense of the barrel for extra weight and complexity of the shells instead. This is most likely to be dealt with by using a mixed battery - heavy long guns or carronades in the lower gun decks, and spigot mortars on the upper deck.

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