User blog comment:GreyFang82/Failed Calculation Test/@comment-25603146-20191029024904

I think that you could just have it such that Asta moves one stride (or, 31 inches, roughly: 0.7874 meters) in the time it takes the bullet to reach him, since the still-framing doesn't really offer a good way to measure it otherwise.

Bullet Speed: 928 m/s

Bullet Moves 7.24 meters in the time that Asta moves 0.7874 meters

928 / (7.24 / 0.7874) = 100.926 meters/second, or Mach 0.294. Which'd be Subsonic.