The easiest way to think about it is in terms of conservation of energy. All the kinetic energy the bullet has at the muzzle is converted into gravitational potential until velocity hits zero. At that point, gravitational velocity starts getting converted directly back into kinetic energy. If not for air resistance, the bullet would reach the ground at roughly muzzle velocity. However, air resistance on the way up zaps some of that energy and presents a velocity barrier on the way down.
The vertical component is primarily determined by sectional density. Tumbling bullet spends a lot of time flying sideways, so its terminal velocity (where air resistance equals weight) is way less than muzzle velocity.
If not fired fairly straight up the bullet can maintain a ballistic trajectory[2], rather than being in a vertical free fall. In that case, the higher speed compared to the terminal velocity is simply because it has yet to shed the speed it acquired[3] when it was accelerated out of the barrel.
[1]: https://en.wikipedia.org/wiki/Terminal_velocity
Actually, it's not a force either[0].
It feels like a force, but it's just mass acting in warped space-time.
[0] http://curious.astro.cornell.edu/about-us/140-physics/the-th...
If you believe in quantum mechanics or some other theory of gravity, this isn't true.
https://bohring.substack.com/p/how-do-we-know-that-general-r...
https://en.wikipedia.org/wiki/Tests_of_general_relativity
https://www.researchgate.net/post/Why_you_think_that_general...
https://www.livescience.com/black-hole-image-shows-einstein-...
Und so weiter, und so weiter...