Snowstar

Mark is a genius military weapon designer who was kidnapped by a mafia gang after refusing to work for them.
They forced him to design unique weapons for five years, until one day he successfully escaped only to get shot by his trusted comrades.
*Ding! You awakened a weapon system.
When he opened his eyes, he was in a different body and in a different world.
"You want to kill a wild boar without being detected? Forget about bow and arrow. Here, I present you this sniper rifle."
"You want to kill a horde of goblins from the sky? Forget about those flying carpets and brooms. Here, I present you the fighter jet." 
"You want to kill a dragon? Forget about taking the help of the S-class adventure team. Here, I present you a nuclear bomb."
Join Mark Spencer/ Lu Zhen as he brings the whole world to its knees with the help of his weapon system.
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Note: Author isn't born in an English-speaking nation and has no editor either. So, please be understanding if there are any grammatical errors.
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Author's current works: War Online, My Soul card is a Reaper
Author's other notable works:  The Last Slytherin, The Sharingan Hyuga
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You can also support the Author through Patreon

https://www.patreon.com/snowstar

Weapon seller in the world of magic

Continuous gunshots reverberated across the hills located outside of Helius city and people who were kneeling on the ground started collapsing with all of their skulls pierced by a 12.7mm round, killing them instantly.

After the last kneeling guy died, the flying horse slowly descended onto the ground and the 12-year-old kid, who was the shooter, jumped from the back of the horse with the sniper rifle.

A middle-aged-looking fellow landed behind him and started clapping, "Bravo, Your Majesty. You are indeed a once-in-a-century genius. Without any formal training, you have not only struck the target without fail but even managed to put a hole in their skulls precisely."

Meanwhile, the soldiers, who were nervous as hell as they stood behind the prisoners as the kid fired the bullets, couldn't help but wipe out their sweat and let out a series of sighs of relief as the mass execution is finished.

Ouyang Zen, the king of Eastern Sun