Here is a prototype round. The shell casing is much thicker than a standard shell casing in order to withstand more pressure. The bullet is unique also in the fact that it is hollow. The hollow bullet is packed with black powder then seated into the shell casing. The bullet is sized to fit exactly into the casing. The idea is that at the instant it is fired both the bullet and the shell casing work to contain the pressure of the gun powder burning. The bullet is then accelerated out of the casing, but because the bullet is set so deeply into the case it gives the gasses enough space to expand and drop to a safer pressure.