FOR REFERENCE:
2x gravity = 2 g, which is apparently 2x9.85 on this sekai rather than Earth's 9.81 m/s2
At 3 minutes (180 s) under upwards acceleration of 2 g, it would reach a velocity of -19.8 x 180 =3564 m/s
For reference standard atmospheric pressure on the Earth is due to the distance to the boundary of space. For us it is 100 km. Since there is no mention of atmospheric pressure, we can assume it is equal to Earth's atmospheric pressure --> 100 km to the border of space. The spear went into space. Unless for this sekai the atmospheric pressure is 10 x that of Earth. Which means a MUCH higher air density, too.
Anyway at the end of 180 s, it will have kinetic energy = (mv^2)/2 = 100 x 3564 x 3564 /2 = 6.35 x 10^8 J
For the spear to turn around, we will apply Energy Conservation Law --> at 1023.5 km it will have1 270 209 600 J in g.p.e.
Total energy of impact = 1 270 209 600 J
Speed of impact = sqrt(25 404 192) = 5040¼ m/s (mach 14.4)