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.
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.
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.
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?