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We have determined mass loss rates and gas expansion velocities for a sample of 69 M-type irregular (IRV; 22 objects) and semiregular (SRV; 47 objects) AGB-variables using a radiative transfer code to model their circumstellar CO radio line emission. We believe that this sample is representative for the mass losing stars of this type. The (molecular hydrogen) mass loss rate distribution has a median value of yr^-1, and a minimum of yr^-1 and a maximum of yr^-1. M-type IRVs and SRVs with a mass loss rate in excess of yr^-1 must be very rare, and among these mass losing stars the number of sources with mass loss rates below a few 10^-8 yr^-1 must be small. We find no significant difference between the IRVs and the SRVs in terms of their mass loss characteristics. Among the SRVs the mass loss rate shows no dependence on the period. Likewise the mass loss rate shows no correlation with the stellar temperature …