  We use two-dimensional hydrodynamical simulations to investigate the
properties of dense ejecta clumps (bullets) in a core collapse supernova
remnant, motivated by the observation of protrusions probably caused by clumps
in the Vela supernova remnant. The ejecta, with an inner flat and an outer
steep power law density distribution, were assumed to freely expand into an
ambient medium with a constant density, $\sim 0.1$ H atoms cm$^{-3}$ for the
case of Vela. At an age of $10^4$ yr, the reverse shock front is expected to
have moved back to the center of the remnant. Ejecta clumps with an initial
density contrast $\chi \sim 100$ relative to their surroundings are found to be
rapidly fragmented and decelerated. In order to cause a pronounced protrusion
on the blast wave, as observed in the Vela remnant, $\chi \sim 1000$ may be
required. In this case, the clump should be near the inflection point in the
ejecta density profile, at an ejecta velocity $\sim 3000 \kms $. These results
apply 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. Radio
observations support this picture, but the possible presence of a compact
object argues against it. The Ni bubble effect or compression in a pulsar wind
nebula are possible mechanisms to produce the clumping.
