Mechanics gives an indication how could weaken fatal speed slew speed.
This can be achieved if many times to lengthen the barrel of the gun.
The extension will require, however, considerable, if we want at the moment of shooting power "artificial" gravity of the projectile was equal to ordinary gravity on earth. A rough calculation shows that this would make the gun length is neither more nor less - 6000 km! In other words, Jules-vernova words should extend into the interior of the globe to the very center... Then the passengers could be spared from the troubles: to their normal weight added would still only the same apparent weight due to a slow slew rate, and they would feel that they were just twice as hard.
However, during the brief period of the human body is capable of without harm to transfer the increase of gravity several times. When we are coming down with icy mountains down here and quickly change the direction of its movement, the weight of our in this brief moment is considerably increased, i.e. our body pressed against Sangam stronger than usual. The increase of gravity three times tolerated us quite well. If we assume that people can safely move within a short period of time, even a tenfold increase in weight, it would be enough to cast cannon "only" 600 km in length. However, this is little consolation, because such a construction lies beyond technically possible.
That's when only the conditions conceivable implementation tempting project Jules Verne: to go to the moon in a cannon shell.*
* In the novel describing the living conditions inside the flying cannon projectile, Jules Verne has made a significant omission, which is described in the first book, "Entertaining physics". The novelist has not taken into account that after the shot at all flight items inside the shell will be completely weightless, because the force of gravity tells the same acceleration and projectile and all bodies in it (see also the article "the Missing Chapter in the novel of Jules Verne").