  We present a simplified dynamical model of the ``Bullet'' system of two
colliding clusters. The model constrains the masses of the system by requiring
that the orbits of the main and sub components satisfy the cosmological initial
conditions of vanishing physical separation a Hubble time ago. This is also
known as the timing argument. The model considers a system embedded in an
over-dense region. We argue that a relative speed of $4500 \rm km/s$ between
the two components is consistent with cosmological conditions if the system is
of a total mass of $2.8\times 10^{15}h^{-1} M_\odot$ is embedded in a region of
a (mild) over-density of 10 times the cosmological background density.
Combining this with the lensing measurements of the projected mass, the model
yields a ratio of 3:1 for the mass of the main relative to that of the
subcomponent. The effect of the background weakens as the relative speed
between the two components is decreased. For relative speeds lower than $\sim
3700\rm km/s$, the timing argument yields masses which are too low to be
consistent with lensing.
