We used two-dimensional hydrodynamical simulations and one-dimensional radiative transport radiation hydrodynamical simulations to investigate the properties of dense ejecta clumps (bullets) in Type Ia and core collapse supernova remnants, motivated by the observation of protrusions probably caused by clumps in Tycho…s remnant (SN 1572) and the Vela supernova remnant. The ejecta were assumed to freely expand into an ambient medium with a constant density. Ejecta clumps with an initial density contrast 100 relative to their surroundings are found to be rapidly fragmented and decelerated. In order to cause a pronounced protrusion on the blast wave front of the remnant as observed in the Vela remnant, a density contrast of ~1000 may be required. This result applies to moderately large clumps; smaller clumps would require an even larger density contrast. Clumps can create ring structure in the shell of the Vela remnant and we investigate the possibility that RX J0852--4622, an apparent supernova remnant superposed on Vela, is actually part of the Vela shell. The heating from the radioactive decay of Ni56 -> Co56 -> Fe56 in supernova ejecta during the first ~10 days is a possible mechanism to produce the clumping, which induces a forward shock that compresses the ejecta gas into a shell. When the flow tends toward a freely-expanding state, the thickness of the shell takes up < 0.5% of the radius of the bubble, and the density contrast across the shell reaches >~ 100 in a narrow region limited by numerical resolution. The resultant interaction of the clumps with the remnant is expected at a stage as indicated by our simulations.