air resistance will be the limiting factor. i am unsure of the actual speed, depends on the bullet weight and size. i do know people who have been hit by falling birdshot (myself included, i do a lot of skeet shooting) and its nothing. but those pellets are smaller.
those kalashnikovs fire a heavy bullet. but were they indeed fire straight up? ive seen most fired maybe 30-40 degrees from vertical but thats just what i see on the news. not doubting you but all it takes is a slightly more horizontal trajectory and it will still have a lot of energy when the bullet hits the ground, as it wouldnt of expended its initial energy gaining altitude.
It seems you are right to feel some caution:
https://science.howstuffworks.com/fire--bullet-straight-up-how-high-does-it-go.htm'deaths not uncommon'
interesting info, thanks for the link (i followed a couple more from that one too).
kalashnikovs are the goto weapon for large parts of the world, and they do back a wallop with a heavy bullet. getting hit with a heavy bullet that size would not be pleasant at all.
i mainly carry a handgun in the woods and thats what i would be firing in the air as a warning shot for the wild dogs and such. and even though i know its miles of woods i think i will at least consider trying to fire into a tree from now on. its just habit to never, ever fire a round with no real target as if i miss the tree that bullet can go a long way. and while there should be no one in my woods.. still..