What else did you expect from this anime?
Willy has courteously provided us with this gif in which the feat is explicitly laid out.
We see the bullet travel between her bust and come back out without even touching her.
Saeko's height is 1.74m according to the wiki, and one's head is generally an eighth of his/her entire body.
((1.74/8)/128)32=0.054375m
Takashi is shooting a Springfield M1A1 Super Match Rifle which fires 7.62mm rounds.
(0.00762/87.24)294.478=0.026m
0.054375-0.026=0.028375m
http://www.gunshopfinder.com/springfieldrifles.html
The chart at the bottom has muzzle velocity at 2800fps, or 853.44m/s.
0.028375/853.44=0.0000332477971504s
((1.74/8)/245)173=0.154m
Saeko's left breast travels roughly half of this.
0.154/2=0.077m
0.077/0.0000332477971504=2316m/s
or about mach 6.8.
I regret this :sanji

Willy has courteously provided us with this gif in which the feat is explicitly laid out.
We see the bullet travel between her bust and come back out without even touching her.

Saeko's height is 1.74m according to the wiki, and one's head is generally an eighth of his/her entire body.
((1.74/8)/128)32=0.054375m

Takashi is shooting a Springfield M1A1 Super Match Rifle which fires 7.62mm rounds.
(0.00762/87.24)294.478=0.026m
0.054375-0.026=0.028375m
http://www.gunshopfinder.com/springfieldrifles.html
The chart at the bottom has muzzle velocity at 2800fps, or 853.44m/s.
0.028375/853.44=0.0000332477971504s

((1.74/8)/245)173=0.154m
Saeko's left breast travels roughly half of this.
0.154/2=0.077m
0.077/0.0000332477971504=2316m/s
or about mach 6.8.
I regret this :sanji