PROJ VEL MULT
defineprojectile <tilenum> PROJ_VEL_MULT <velocity multiplier>
PROJ_VEL_MULT is a defineprojectile property used to set multiply velocity of a projectile (causes an artificial acceleration).
On a given tic the projectile will move PROJ_VEL_MULT times, for example, if you set PROJ_VEL_MULT to 1, it will only move at its current velocity (stored in its sprite's xvel / zvel) once per tic.
Where <velocity multiplier> is the amount to multiply by. See also PROJ_VEL.