Both...
In a semi auto, an uncrimped bullet "can" experience set-back, i.e. get pushed back in its case somewhat by the mechanical motion of stripping and loading of the round (as the bullet tip contacts a feed ramp for example).
In a revolver it is a different story. Since the rim is firmly held from moving forward, and the heavy bullet is not, the cylinder can act like a kinetic bullet puller, and actually pull the bullet out of the case some, leading to jams.
- AG