King Sejong praised the elite of the artillery soldiers before getting to the main point.
"Is the cannon they are carrying a newly made one?" At Sejong's question, Hyang immediately replied, "Yes, it is. We have named it the Gapsikjangtong."
Sejong stroked his beard thoughtfully as he expressed his admiration.
"A long cannon... it suits its name well. But..."
Sejong's gaze was fixed on the gleaming barrel of the long cannon.
Looking at the silver-shining barrel, Sejong turned to Hyang.
"It is not made of copper, is it?"
"Indeed, it is made of siusoe (soft iron)."
"Siusoe?" Huang He, who had been listening to the conversation between Sejong and Hyang, interjected immediately.
"Your Majesty, while copper is expensive, siusoe is also costly..."
Sejong raised his hand to interrupt Huang He.
"Enough, the Crown Prince must have his reasons for using siusoe. We can discuss that later."
"Understood."
After stopping Huang He, Sejong gave an order to Choi Haesan.
"I want to see its power."
"Yes, Your Majesty."
Following the command, Choi Haesan gestured to the artisans beside him.
The artisans, upon seeing Choi Haesan's signal, soon brought forth a target board the size of a human torso.
The board, covered with red cloth, had several holes drilled into it.
Choi Haesan pointed to the board with his baton and began to explain.
"This is the target board we will use. Compared to the actual armor used, we have increased the thickness of the iron plate by 1 ri, making it 4 ri (about 1.3mm) thick."
"Is that so?"
"Let’s set up the target."
"Yes, General."
The artisans took the target to the opposite side of the demonstration area.
Seeing the target being set up far from where the artillery soldiers were stationed, Sejong called out to General Choi Haesan.
"General, how far is it to there?"
"About 250 bo (approximately 300m)."
"Ho!" Sejong's eyes sparkled with admiration.
Meanwhile, after finishing the target setup, the artisans signaled each other and quickly dashed to hide in a trench made beneath the earthen wall.
Seeing the artisans' signal, the commander shouted to the artillery soldiers.
"Stand the cannons!"
At the command, the artillery soldiers stood their long cannons upright on the ground.
"Load the cannon!"
"Insert the gunpowder!"
"Insert the paper!"
"Insert the projectile!"
"Insert the lead!"
Following the commander's orders, the artillery soldiers completed all the steps and returned to the standing position.
The commander then issued the command.
"Aim the cannon! Fire!"
The artillery soldiers lifted their long cannons, pulled the firing mechanism halfway back, and inserted the copper projectile.
"Pull the firing mechanism! Aim!"
The artillery soldiers fully pulled back the firing mechanism, stepped forward with their left foot, tilted their bodies, and aimed at the target standing far away.
"Fire!"
Boom! In an instant, a cloud of smoke and a burst of flame erupted, disappearing into the air.
"Right on target!"
After firing, the artillery soldiers returned to the standing position, and the artisans who had hidden in the trench came back to retrieve the target.
"Impressive!"
"What tremendous power!" The plump Sejong and the ministers could not hide their astonishment as they examined the retrieved target board, which had nine holes drilled into it.
Seeing the astonished faces of Sejong and the ministers, Hyang and Choi Haesan were filled with pride.
"Truly remarkable. As I understand, while cannons are powerful, their range is far inferior to that of a bow. However, this long cannon has a range similar to that of a bow, and its power is unmatched, making it truly the strongest weapon."
"The artisans have worked hard."
In response to Sejong's praise, Hyang humbly replied.
Hearing Hyang's response, Choi Haesan nodded vigorously.
Tears welled up in Choi Haesan's eyes as he nodded.
Noticing Choi Haesan's intense reaction, Sejong turned to Hyang.
Seeing Hyang's face, which wore a somewhat embarrassed smile, Sejong sighed.
"Just how much hardship have the artisans endured... General Choi, tell me about the process of completing this cannon."
"Yes, Your Majesty. So..."
The beginning of making the long cannon started with creating specialized tools to form the iron body of the cannon.
"To make the cannon, we must use a specialized anvil," Hyang commanded.
Following his orders, the artisans made a dedicated anvil using high-carbon steel.
While a regular anvil has a flat top, the specialized anvil had a long groove shaped like a large semicircular column, gradually tapering to a size slightly larger than the intended thickness of the cannon.
Once the anvil was completed, Hyang ordered an iron core made of high-carbon steel, slightly thicker than the cannon's specifications.
After the specialized tools were made, Hyang directed the next steps.
Under Hyang's guidance, the artisans of the military equipment department began to roll out appropriately sized plates of siusoe.
Once the iron plates were rolled to the right thickness, they inserted the iron core in the center and began to hammer the plates on the specialized anvil.
As the plates were rolled, the ends met, and the artisans sprinkled a mixture of iron powder and charcoal powder on the joint before placing it in the furnace.
The hot heat of the furnace melted the mixture, filling the wrinkles of the rolled iron plate.
Once the gaps were completely filled, the artisans smoothed it out with a file while checking if it was properly sealed.
If everything was fine, they hammered the round piece into a suitable shape for future processing, making it easier to work with the cannon.
Finally, after heating the cannon in the furnace and burying it in ash to cool, Hyang used a drilling tool he devised to smooth the interior.
Once the inside was polished smoothly and both the exterior and interior of the cannon were confirmed to be straight without any bends, the artisans filed the barrel to have an octagonal cross-section.
After that, they drilled a female screw at the back and inserted a cap with a male screw—this part was where the copper projectile would be fitted.
The work did not end there.
The trigger and firing mechanism were attached to the wooden body made by the woodworker, and the trigger guard and sight, made by the metalworker, were installed.
The end of the muzzle was reinforced with a solid piece, and protruding parts were fitted on both sides.
The real struggle began then.
The barrel designed by Hyang did not have rifling.
This was Hyang's intention.
"As we continue to create and test new items, I cannot be the one to handle everything," he thought.
Once five prototypes were completed, Hyang, along with Choi Haesan and Lee Cheon, began the performance tests in earnest.
"First, let’s find out how much gunpowder it can withstand."
"Yes, Your Majesty."
"The existing cannon can handle one yang, so let’s start with half a yang."
"That seems appropriate."
Choi Haesan, Lee Cheon, and the artisans began the test by filling the cannon with half a yang of Gapsik gunpowder and projectiles.
For safety, they secured the cannon to a fixture, connected a string to the trigger, and hid behind a protective wall.
"Fire! Fire! Fire!"
Boom! The test filled the area with a loud explosion, concluding that the cannon could withstand up to one and a half yang of Gapsik gunpowder.
"Considering the purpose of this cannon, it seems we do not need to go up to two yang."
"Indeed."
"Then let’s try it with Eulsik gunpowder."
In the subsequent test, the long cannon also withstood one and a half yang of Eulsik gunpowder.
After confirming the results, Hyang decided to change the test parameters.
"Let’s find out the optimal amount of gunpowder. The distance at which the existing cannon killed a soldier in armor was said to be 40 bo (about 50m), so let’s use that as a reference."
"Yes, Your Majesty."
This time, they would reduce the amount of gunpowder.
The result of the test was two don (錢, about 7.5g) of Eulsik gunpowder.
The projectile fired from the long cannon cleanly penetrated the 4 ri thick iron plate used as the target.
After confirming the results, Hyang looked at Choi Haesan and Lee Cheon.
"Given the power, it seems we could kill from even further away. The farther, the better, right?"
"Of course."
"Then shall we see if it can reach 50 bo?"
"Yes, Your Majesty."
Choi Haesan replied excitedly.
Thus, in the ongoing tests, the long cannon loaded with two don of Eulsik gunpowder penetrated the target at a distance of 70 bo (about 85m).
Beyond that distance, while the projectiles reached, none hit the target, despite firing over twenty shots.
"70 bo is unexpected. It seems it’s time to start hinting at the rifling."
Though the results exceeded his expectations, Hyang had no intention of stopping.
From that point on, however, Choi Haesan, Lee Cheon, and the artisans found the work less enjoyable.
"General, observing the loading process of the cannon, it seems that the time spent on loading and firing is excessive. Let’s try without loading and firing."
"But without that process, the projectile might fall out..."
"Instead, we could make the projectiles fit the cannon perfectly, couldn’t we?"
After a moment of contemplation, Choi Haesan nodded at Hyang's suggestion.
"It seems worth testing."
Hyang planned to force them to experience trial and error based on a video he had seen online.
In the experiment that followed, the long cannon fired five shots before encountering a problem.
"The residue from the gunpowder is making it difficult to prepare for the next shot."
"That is true."
"It seems we must load it..."
However, Hyang shook his head.
"General, think about it. When cavalry rides horses at full speed, do they have the luxury to load?"
"That is true, but..."
Choi Haesan, Lee Cheon, and the artisans began to brainstorm solutions, but they struggled to think of rifling.
"Should I step in again?" On a rainy day, while pondering whether to intervene, one artisan suddenly stood up from his seat, staring blankly out the window.
"Ah!"
"What is it?"
"How about we carve grooves?"
"Carve grooves?" The artisan pointed to the ground with his finger to explain.
"Look at that trench. The stagnant water flows out of that trench, doesn’t it? The flow of gunpowder flames should be similar, right? If we carve long grooves inside the cannon, wouldn’t the residue flow out along with the flames?"
At the artisan's words, Choi Haesan and Lee Cheon looked at Hyang.
Hyang nodded.
"It seems worth testing."
Following Hyang's decision, seven additional long cannons were produced.
"Let’s create two of each cannon with three, six, nine, twelve, and fifteen grooves and compare them with the un-grooved cannons."
"Yes."
Though they received the order, the artisans faced a new dilemma.
"How should we carve the grooves?" After several sleepless nights of deliberation, specialized tools were created, and the artisans carved three to fifteen grooves into the long cannons.
In the subsequent tests, Choi Haesan, Lee Cheon, and the artisans encountered a new problem.
The more grooves there were, the weaker the power became.
"That is to be expected. The more grooves there are, the more force is lost along those grooves," Lee Cheon pointed out the cause, and Choi Haesan muttered with a troubled expression.
"Is it really only loading that is the answer?"
"What were the exact results?" At Hyang's question, Lee Cheon flipped through a book with the results.
"Up to six grooves, the power loss is minimal, but there is a lot of residue. Starting from nine grooves, the residue decreases, but the power drops sharply."
Upon hearing Lee Cheon's response, everyone furrowed their brows.
"Should we go with nine grooves..."
"The problem is that the power visibly decreases from that point..."
"Then is six grooves the only answer? But that also has residue issues..."
Seeing the group struggle with problems that could not be solved simultaneously, Hyang was about to speak when Jang Yeong-sil spoke up.
"How about twisting the grooves?"
"Twisting the grooves?" All eyes turned to Jang Yeong-sil at his suggestion.
Hyang, who had been watching from behind, clenched his fist under the table.
"Nice one, uncle!"
Quickly composing himself, Hyang commanded Jang Yeong-sil.
"Master Jang, please explain properly."
"Yes, Your Majesty. So... if we twist the grooves like a screw, wouldn’t the length increase? Even with six grooves, it would provide enough length for the residue to escape, and since it twists, the force of the explosion wouldn’t escape immediately, right?"
Jang Yeong-sil's explanation plunged the meeting room into silence.
After a long moment of contemplation about what Jang Yeong-sil meant, Choi Haesan and Lee Cheon shouted in unison.
"That’s it!"
"That’s it! It seems that twisting is the answer!"
With Choi Haesan and Lee Cheon's words, Hyang reached a conclusion.
"Then let’s try it."
That night, as he lay down to sleep, Hyang muttered with a tired expression, "Ah... the road to rifled cannons is indeed long and arduous... explaining the concept of surface area in terms of length... and how do they understand it so well... are all the people living in this era monsters?"
The decision to twist the grooves inside the cannon was made, but another problem arose.
"How many twists should we make?" Faced with this question, the group once again found themselves at a loss, prompting Hyang to step in once more.
"Make prototypes with half a twist to two twists in half-twist increments!"
"Understood!"
And so, the artisans once again stayed up all night to create new long cannons.