12 gauge shotgun, and go for 00 buckshots. It came in 9 small pellets but it spread out like fireworks, and since you aim the gun right at your chest skin right then no need slug. The buckshots do fine since it skin to gun distance.
Buckshot has 9 pellets, so those pellets will spread out as you fire the gun, one of them got to hit your heart, or at least will blast your chest.
When you aim far, you need big pellet like slug. But skin to skin, buckshot is fine.
Shotgun is my 2nd CTB choice, I would point right at my temple and blast it though.