  One of the two outcomes of a common envelope event is a merger of the two
stars. To date, the best known case of a binary merger is the V1309 Sco
outburst, where the orbital period was known and observed to decay up to the
outburst. Using the hydrodynamical code StarSmasher, we study in detail which
characteristics of the progenitor binary affect the outburst and produce the
best match with observations. We have developed a set of tools in order to
quantify any common envelope event. We consider binaries consisting of a
1.52$M_\odot$ giant and a 0.16$M_\odot$ companion with $P_{\rm orb}\sim1.4$
days, varying the nature of the companion and its synchronization. We show that
all considered progenitor binaries evolve towards the merger primarily due to
the Darwin instability. The merger is accompanied by mass ejection that
proceeds in several separate mass outbursts and takes away a few percent of the
donor mass. This very small mass, nonetheless, is important as it is not only
sufficient to explain the observed light-curve, but it also carries away up to
1/3 of the both initial total angular momentum and initial orbital energy. We
find that all synchronized systems experience $L_2$ mass loss that operates
during just a few days prior to merger and produces ring-shaped ejecta. The
formed star is always a strongly heated radiative star that differentially
rotates. We conclude that the case of a synchronized binary with a
main-sequence companion gives the best match with observations of V1309 Sco.
