Well, no gun today has a muzzle velocity of 17,000 MPH, needed to escape earths gravity.
Guns with the longest range are 25 miles at best, and that is not even half way out of the atmosphere.
Now the US navy has been playing with the rail gun, In tests, projectiles are out to 100 miles, and mach 7 velocity, that would stand a chance of putting a bullet into orbit.
Somewhere I did read of one specialized gun, R&D at Sandia labs shooting a projectile as fast as 10 miles per second,,I would think a man made meteorite and burn up in the atmosphere.
NASA has long been using air guns simulating micro meteorites.