  We investigate the role played by initial clumping of ejecta and by efficient
acceleration of cosmic rays (CRs) in determining the density structure of the
post-shock region of a Type Ia supernova remnant (SNR) through detailed 3D MHD
modeling. Our model describes the expansion of a SNR through a magnetized
interstellar medium (ISM), including the initial clumping of ejecta and the
effects on shock dynamics due to back-reaction of accelerated CRs. The model
predictions are compared to the observations of SN 1006. We found that the
back-reaction of accelerated CRs alone cannot reproduce the observed separation
between the forward shock (FS) and the contact discontinuity (CD) unless the
energy losses through CR acceleration and escape are very large and independent
of the obliquity angle. On the contrary, the clumping of ejecta can naturally
reproduce the observed small separation and the occurrence of protrusions
observed in SN 1006, even without the need of accelerated CRs. We conclude that
FS-CD separation is a probe of the ejecta structure at the time of explosion
rather than a probe of the efficiency of CR acceleration in young SNRs.
