After praising the elite soldiers' cannons, Sejong proceeded to the main topic.
"Are the cannons they carry on their shoulders the newly made ones?"
In response to Sejong's question, Hyang immediately answered.
"Yes, Your Majesty. We named them Gapseujangchong (甲式長統筒)."
Sejong stroked his beard and shared his thoughts.
"Gapseujangchong... It's a fitting name. However..."
Sejong's gaze was fixed on the gleaming barrels of the Gapseujangchong. Looking at the silver-shining barrels, Sejong turned to Hyang.
"They're not made of copper, are they?"
"That is correct, Your Majesty. They are made of steel."
"Steel?"
Hwang Hee, who had been listening to the conversation between Sejong and Hyang, interjected.
"Your Majesty, copper is expensive, but steel is also... costly."
Sejong raised his hand to cut off Hwang Hee.
"Enough. The Crown Prince must have had his reasons for using steel. We can discuss this later."
"As you command, Your Majesty."
Sejong, who had stopped Hwang Hee, gave an order to General Choe Hae-san.
"I want to see its power."
"As you command, Your Majesty."
Upon receiving the order, General Choe Hae-san gestured to the artisans standing beside him. The artisans quickly brought a target board about the size of a human torso. The red cloth-covered board was studded with iron plates. General Choe Hae-san pointed to the board with his baton and began to explain.
"This is the target board. Compared to the actual iron plates used, the thickness of the iron plates on this board has been increased by 1 li to 4 li (approximately 1.3 mm) for reinforcement."
"Is that so?"
"Set up the target."
"As you command, General."
The artisans carried the target to the opposite end of the demonstration area. Seeing the target being set up far from where the cannon soldiers were standing, Sejong called out to General Choe Hae-san.
"General, how far is it from here to there?"
"About 250 paces (approximately 300 meters), Your Majesty."
"Wow!"
Sejong's eyes lit up with amazement. Meanwhile, the artisans who had finished setting up the target signaled and quickly ran to the side, diving into a trench below the earthen wall.
Seeing the signal from the artisans, the commander shouted to the cannon soldiers.
"Stand your cannons!"
Following the commander's order, the cannon soldiers stood their Gapseujangchong on the ground.
"Clean the cannons!"
"Load the gunpowder!"
"Insert the paper!"
"Load the projectile!"
"Insert the lead ball!"
After completing all the steps in response to the commander's commands, the soldiers returned to the standing position, and the commander gave the final order.
"Ready your cannons! Attach the Donghwamo (銅火權)!"
The cannon soldiers lifted their Gapseujangchong, pulled the flintlock halfway, and attached the Donghwamo, which contained the primer, to the nipple.
"Pull the flintlock! Aim!"
The cannon soldiers pulled the flintlock all the way back, stepped forward with their left foot, leaned their bodies slightly, placed the butt against their shoulders, and aimed at the distant target.
"Fire!"
Boom!
Instantly, a thick smoke and a flash of fire erupted and disappeared into the sky.
After the firing, the cannon soldiers returned to the standing position, and the artisans who had been hiding in the trench came back to retrieve the target.
"Amazing!"
"Truly formidable power!"
Sejong and the ministers, inspecting the recovered target board, could not hide their surprise.
The target board had nine holes neatly lined up.
Seeing the amazed faces of Sejong and the ministers, Hyang and General Choe Hae-san felt a sense of pride.
"Truly remarkable. As I understand it, cannons are powerful but have a much shorter range compared to bows. However, this Gapseujangchong has a range similar to a bow, and its power is incomparable. It is truly the strongest weapon."
"The artisans have worked hard."
In response to Sejong's praise, Hyang humbly replied. General Choe Hae-san, who had heard Hyang's response, nodded vigorously. Tears welled up in his eyes as he nodded.
Seeing General Choe Hae-san's intense reaction, Sejong turned to Hyang. Seeing Hyang's face with a slight smile, Sejong sighed.
"How much have you made the artisans suffer... General Choe. Tell me about the process of completing this cannon."
"As you command, Your Majesty. Well..."
* * *
The process of making the Gapseujangchong began with creating a specialized tool for forging the iron body of the cannon.
"To make the cannon using forging, we need a specialized anvil."
Following the king's order, the artisans made a specialized anvil using high-carbon steel. Unlike a regular anvil, which has a flat top, the specialized anvil had a long groove shaped like a series of half-cylinders, gradually decreasing in size from a large half-cylinder to one slightly larger than the intended cannon's diameter.
After the anvil was completed, Hyang ordered the creation of an iron mandrel slightly thicker than the cannon's bore, also made of high-carbon steel.
With the specialized tools ready, Hyang guided the next steps.
Following Hyang's guidance, the chief artisans, including Go Ya-jang, began rolling a steel plate to the appropriate size.
Once the steel plate was rolled to the right size, they inserted the iron mandrel and started hammering the plate on the specialized anvil to form it into a tube. As the plate was hammered into a tube, the ends met, and the artisans sprinkled a mixture of metal powder and charcoal on the seam before placing it in the furnace. The heat from the furnace melted the mixture, filling the seam and making the tube smooth and round.
Once the seam was completely filled, the chief artisans checked it with a string to ensure it was smooth and properly sealed. If it was sealed correctly, they hammered the tube to form an octagonal cross-section.
This was to facilitate the subsequent processing steps, making it easier to secure the cannon for work.
Finally, after heating the cannon in the furnace and cooling it in ashes, it was polished inside using a boring machine designed by Hyang.
Once the inside was polished smooth and the cannon was confirmed to be straight both inside and out, the artisans wrapped the barrel with a string to form an octagonal cross-section. They then drilled a female thread at the rear and attached a cap with a male thread, where the Donghwamo would be inserted.
The work did not end there. The wooden stock, made by the chief woodworker, was combined with the trigger and flintlock mechanism, and the chief metalworker added the trigger guard and sight. A solid part with protrusions on both sides was attached to the muzzle. The hard work began from there.
Seeing the completed prototypes, Hyang, along with General Choe Hae-san and Yi Cheon, began the formal performance tests.
"Let's first find out how much gunpowder it can withstand."
"As you command, Your Majesty."
"Since the existing cannons can handle 1 nyang of gunpowder, let's start with half a nyang."
"That seems appropriate."
General Choe Hae-san, Yi Cheon, and the artisans began the test by loading half a nyang of gunpowder and a projectile into the cannon.
For safety, the cannon was secured in a mounting, and a rope was attached to the trigger. The people then hid behind a protective wall.
"Fire! Fire! Fire!"
Boom!
The test, which filled the 51st area with a loud explosion, ended with the cannon withstanding 1.5 nyang of gunpowder.
"Considering the purpose of this cannon, 2 nyang is probably unnecessary."
"You are correct."
"Then let's try with Eui-style gunpowder."
In the subsequent test, the Gapseujangchong also withstood 1.5 nyang of Eui-style gunpowder. After confirming the results, Hyang changed the test item.
"Let's determine the optimal amount of gunpowder. The distance at which the existing cannons killed a Jurchen warrior wearing iron armor was 40 paces (approximately 50 meters). Let's use that as a reference."
"As you command, Your Majesty."
This time, they reduced the amount of gunpowder. The test results showed that 2 jeon (錢, approximately 7.5 grams) of Eui-style gunpowder was sufficient. The projectile fired from the Gapseujangchong cleanly penetrated the 4-li thick iron plate used as a target.
After confirming the results, Hyang looked at General Choe Hae-san and Yi Cheon.
"Judging by the power, it seems possible to kill at a greater distance. The farther, the better, right?"
"Of course."
"Then, let's see if it can do 50 paces."
"As you command, Your Majesty."
General Choe Hae-san answered enthusiastically. In the test, the Gapseujangchong with 2 jeon of Eui-style gunpowder penetrated the target at 70 paces (approximately 85 meters). Beyond that distance, the projectile reached the target, but none of the 20 shots hit the mark.
'70 paces, better than expected? Now, it's time to hint at rifling?'
Despite the better-than-expected results, Hyang was not ready to stop. On a rainy day, as he watched an artisan staring blankly out the window, the artisan suddenly stood up.
"Ah!"
"What is it?"
"What about cutting grooves?"
"Cutting grooves?"
The artisan pointed to the ground with his finger and explained.
"Look at this trench. Doesn't the water flow through this trench? The flame from the gunpowder also has a flow, right? If we cut long grooves inside the cannon, wouldn't the residue also flow out along the grooves?"
Hearing the artisan's suggestion, General Choe Hae-san and Yi Cheon looked at Hyang. Hyang nodded.
"It seems worth testing."
Following Hyang's decision, seven more Gapseujangchong were made.
"Let's make two cannons each with 3, 6, 9, 12, and 15 grooves and compare them with the ones without grooves."
"As you command."
Although they received the order, the artisans had a new challenge.
'How do we cut the grooves?' After several sleepless nights, a specialized tool was created, and the artisans cut the grooves in the cannons, ranging from 3 to 15.
In the subsequent tests, General Choe Hae-san, Yi Cheon, and the artisans encountered a new problem: the more grooves there were, the less powerful the cannon became.
"Of course, the more grooves, the more force leaks out."
As Yi Cheon pointed out the cause, General Choe Hae-san muttered with a troubled expression.
"Is reloading the only solution?"
"What are the exact results?"
In response to Hyang's question, Yi Cheon flipped through the book with the results and answered.
"Up to 6 grooves, the power reduction is minimal, but there is a lot of residue. From 9 grooves, the residue decreases, but the power drops sharply."
Hearing Yi Cheon's answer, everyone was at a loss.
"Should we go with 9 grooves..."
"The problem is that the power visibly decreases from 9 grooves..."
"Then, 6 grooves? But that has the residue issue..."
Seeing the artisans struggling with these unsolvable problems, Hyang was about to speak when Jang Yeong-sil spoke up.
"What about spiraling the grooves?"
"Spiraling the grooves?"
Everyone's attention turned to Jang Yeong-sil.
Hyang, who had been watching from the back, clenched his fist under the table.
'Uncle, nice one!'
Quickly composing himself, Hyang ordered Jang Yeong-sil.
"Jang Byeol-ja, please explain in detail."
"As you command, Your Majesty. Well... If we spiral the grooves like the screws we use, wouldn't the length increase? This way, even with 6 grooves, there would be enough length for the residue to flow out, and the force from the gunpowder explosion would not escape directly due to the spiraling."
The room fell silent as everyone pondered Jang Yeong-sil's explanation. After a while, General Choe Hae-san and Yi Cheon shouted simultaneously.
"That's it!"
"That's it! Your Highness, spiraling is the answer!"
Hearing General Choe Hae-san and Yi Cheon, Hyang made a decision.
"Let's try it."
That night, as Hyang lay in bed, he grumbled with a tired expression.
"Ah... The path to rifling is truly long and arduous... Explaining the concept of surface area in terms of length... And the ability to understand it so easily... Are all the people of this era monsters?"
The decision to spiral the grooves inside the cannon was made, but another problem arose.
"How many turns should we make?"
Faced with this question, people were again at a loss, and Hyang had to step in again.
"Make prototypes with half a turn, one turn, one and a half turns, and two turns!"
"As you command!"
And the artisans had to stay up all night again to make new Gapseujangchong.