Context : In this paper we present the first SiO multiline analysis ( from J =2-1 to J =11-10 ) of the molecular bullets along the outflows of the Class 0 sources L1448-mm and L1157-mm , obtained through observations with IRAM and JCMT .

Aims : This analysis has been performed to investigate the physical properties of different types of bullets , in order to explore their excitation conditions and put constraints on their formation and evolution .

Methods : For this purpose we have computed the main physical parameters ( n _ { H _ { 2 } } , T _ { kin } , N _ { SiO } ) in each bullet and compared them with other tracers of warm and dense gas and with models for the SiO excitation in shocks .

Results : We find that the bullets close to L1448–mm , associated with high velocity gas , have higher excitation conditions ( n _ { H _ { 2 } } \sim 10 ^ { 6 } cm ^ { -3 } , T \ga 500 K ) with respect to the L1157 bullets ( n _ { H _ { 2 } } \sim 1–5 \times 10 ^ { 5 } cm ^ { -3 } , T \sim 100 - 300 K ) .
In both the sources , there is a clear evidence of the presence of velocity components having different excitation conditions , with the denser and/or warmer gas associated with the gas at the higher speed .
In L1448 the bulk of the emission is due to the high-excitation and high velocity gas , while in L1157 most of the emission comes from the low excitation gas at ambient velocity .
The observed velocity-averaged line ratios are well reproduced by shocks with speeds v _ { s } larger than \sim 30 km s ^ { -1 } and densities \sim 10 ^ { 5 } – 10 ^ { 6 } cm ^ { -3 } .
Plane-parallel shock models , however , fail to predict all the observed line profiles and in particular the very similar profiles shown by both low and high excitation lines .

Conclusions : The overall observations support the idea that the L1157 clumps are shock interaction events older than the L1448 bullets close to the driving source .
In the latter objects , the velocity structure and the variations of physical parameters with the velocity resemble very closely those found in optical/IR jets near the protostar , suggesting that similar launching and excitation mechanisms are also at the origin of collimated jets seen at millimetre wavelengths .