  The time resolution of Multigap Resistive Plate Counters (MRPCs) to $511$ keV
gamma rays has been investigated using a $^{22}$Na source and four detectors.
The MRPCs time resolution has been derived from the Time-of-Flight information,
measured from pairs of space correlated triggered events. A GEANT4 simulation
has been performed to analyze possible setup contributions and to support
experimental results. A time resolution (FWHM) of $376$ ps and $312$ ps has
been measured for a single MRPC with four $250$ $\mu$m gas gaps by considering
respectively one and two independent pairs of detectors.
