Poking the pellet into the breech with a probe may well help the tail clear the obstruction, but it also increases the void space between the front end of the piston and the back end of the pellet.
If a fixed amount of air - the swept volume, or close to it - is to be compressed into a small space, then the smaller that space the higher the compression ratio. As this compression takes place so quickly there is little opportunity for the heat created to escape to the metal parts. Look here http://en.wikipedia.org/wiki/Adiabatic_process
for an outline of adiabatic laws.