From what little research I have done due to the comments on this manga, I have come to the conclusion that Japan has extremely low Deaths by Gunshots, as well as an extremely low number of gun holders, and the only guns legally allowed to own are guns for Hunting and Sports Shooting, and even then you have to go through rigorous amounts of testing, questioning, and background checking before possibly being granted a hunting/sports gun. It is thanks to this that, even during full-on Molotov-throwing riots, the police in Japan would often - if not always - prefer more body armor over using their guns.
I have also found that you can't own a gun in Japan unless you are 20 years of age or older and that some statistics claim that 65% of the 190,000 registered Hunters back in 2010 were 60 years or older.
I also found that many have shot targets with special slugs at 100 to 200 yards, and this one specific review article state it is "a 300-yard deer murdering machine." This article's reviewer used an x9 scope for his shooting, as well.
A Trophy Copper Sabot Slug 12 Gauge, - which I believe is a variation of, if not the same as, the 12 gauge federal copper sabot slug mentioned by the protagonist - can do Sub-4 inch groups at 200 yards, and is used in Rifled shotguns.
Sources:
https://www.loc.gov/law/help/firearms-control/japan.php
https://www.tofugu.com/japan/guns-in-japan/
https://www.thetruthaboutguns.com/remington-870-sps-superslug-12-gauge-shotgun-gun-review/
https://www.federalpremium.com/shotshell/premium-slug-buckshot/vital-shok-trophy-copper-sabot-slug/11-P151+TC.html
