[luis8]

What about using a long ship and then using a high tech spring to apply the deceleration over the length of it. Relocate the passenger at the front and repeat until you reached the desired speed.

Just thinking out loud maybe that will be counterproductive in the amount of time needed

[Valgrim]

This reminds me of the device used in Jules Vernes " From the Earth to the Moon". The projectile has "thick lining of leather fastened on springs of best steel", together with some sort of hydraulic system, meant to absorb the shock of shot.