Others have calculated the g-forces to be about 1670 g. Not suitable for anything living.
You mention this is an explosion driven cannon (so very much a traditional one). What makes me wonder is with a 1 km barrel, how are they going to keep up the pressure to continue to accelerate the projectile all the way? That needs a really massive expansion of gases, for long time (0.37 seconds: explosions are normally way faster than that), the gases have to travel all the way to the nozzle together with the projectile, etc.
I do have the feeling that a rail gun kind of design would scale much better.