In fact all bullets drop at the same speed. The same speed they drop when you hold one out to your side and release it.
The only question is how far the bullet traveled in that period of time.
That’s the point. Fast (usually light) bullets are traveling fast enough that they don’t drop much by the time they get to the target, so the drop can usually be ignored or minimally compensated for. Slow (usually heavy) bullets take so long to get there that they fall a significant distance, hence the pronounced arc which must be accounted for.
“In fact all bullets drop at the same speed....”
Nothing falls at the same speed.
Any object dropped near the earth’s surface accelerates - picks up downward velocity - at the rate of 32.17 ft per second, for each second of travel. It does lose velocity to air resistance (minor at low speeds).
At the start of the fall, vertical velocity is zero.
After one second of fall, it is 32.17 ft/sec.
After two seconds, vertical velocity is 64.34 ft/sec, etc.
A bullet fired straight up will lose vertical velocity at the same rate, 32.17 ft/sec each second. Therefore, ignoring air resistance, a bullet traveling straight up at 321.7 ft/sec will slow to zero after ten seconds, then reverse course and start to fall downward, picking up velocity at the same rate.