Mullifications:
A slightly less rifling twist is "needed" in a vacuum because the twist rate required depends on the density of the medium the bullet is going through.
The difference between the density of air (1.0) and the density of a vacuum (zero) is so minimal it would not be necessary to change the rifling twist.
And anyhow, the "calculations" for rifling twist are only approximations --quoting an exact number is unnnecessary, especially since other factors like the form and length and material of the bullet are more important.
In "practice" (heh-heh) all the factors involving orbital speed would cancel out for a shooter and a target in essentially the same orbit for practical ranges and the shooting would pretty much be a straight line affair except possibly for "lead," with no drop calculations needed. In other words, the sights should line up exactly with the axis of the bore to make a hit. The velocity of the bullet in a vacuum would be slightly greater because the bullet does not have to push a column of air out the barrel ahead of it.
For a bullet fired straight "ahead" in the orbit, the projectile would seek a higher orbit and probably continue to orbit in a more elliptical orbit. The chances of getting hit by your own bullet are almost nonexistent, considering the errors involved accumulating over the well-over 25,000 mile flight around the full orbit. For a bullet fired "backward" in the orbit, the bullet would seek a lower orbit. However, this lower orbit might intersect with the earth. Or at least be slowed by the atmosphere and burn up.
If that bullet could be fired "backward" at exactly the orbital velocity, it would fall straight down to the earth, but I know of no gun which can fire at any low earth orbital velocities.
Fun mulling it over, huh?
Terry