  The modified version of a bullet model for gamma ray bursts is studied. The
central engine of the source produces multiple sub-jets that are contained
within a cone. The emission of photons in the source frame of a sub-jet either
takes part in an infinitesimally thin shell, or during its expansion for a
finite time. The analysis of the observed profiles of GRBs taken by BATSE leads
us to the conclusion that the latter possibility is much more favored. We also
study the statistical distribution of GRBs, in the context of their bimodality
of durations, taking into account the detector's capability of observing the
signal above a certain flux limit. The model with shells emitting for a finite
time is able to reproduce only one class of bursts, short or long, depending on
the adopted physical parameters. Therefore we suggest that the GRB bimodality
is intrinsically connected with two separate classes of sources.
