[robotresearcher]

The standard approach is to place the cannon such that no trajectory with the energy of the projectile can hit anything important. That's why huge ranges like the Canadian Forces Base Suffield exist [2,690 km2 (1,040 sq mi)]. The the only dangerous variable is the energy you put in, and not the direction you point the gun, unlucky bounces, etc.

http://en.wikipedia.org/wiki/CFB_Suffield