  The jets of SS 433 have punched through the supernova remnant from the
explosion forming the compact object. The precessing jets collimate before
reaching beyond the shell, some 40 pc downstream, just the region of origin of
TeV gamma radiation. Collimation could be effected by ambient pressure in the
SNR cavity; I investigate conditions under which W50 morphology and the origins
of TeV gamma radiation can be explained in terms of collimation, with
associated shocks, induced by ambient pressure. I consider the age of the SNR
and hence the present ambient pressure. The cavity is now ~ 10^5 years after
the supernova; with lower pressure, collimation and associated shocks would
occur ~40 pc downstream, just the origin of TeV gamma radiation. Modelling of
evolution of binary systems indicates that Roche lobe overflow and initiation
of the jets is recent. Collimation would still occur ~40 pc downstream, but the
cone angle of the precession must then have increased with time - driven by the
Roche lobe overflow. The morphology of W50 and the site of the origin of TeV
gamma radiation are readily explained in terms of collimation of the jets by
internal SNR pressure.
