We start reviewing Giulini's dynamical approach to Bargmann superselection rule proposing some improvements. We discuss some general features of the central extensions of the Galileian group used in Giulini's programme, focussing on the interplay of classical and quantum picture, without making any particular choice for the multipliers. Preserving other features of Giulini's approach, we modify the mass operator of a Galilei invariant quantum system to obtain a mass spectrum that is positive and discrete, giving rise to a standard (non-continuous) superselection rule. The model is invariant under time reversal but a further degree of freedom appears, interpreted as an internal conserved charge. (However, adopting a POVM approach a positive mass operator arises without assuming the existence of such a charge.) The effectiveness of Bargmann rule is shown to be equivalent to an averaging procedure over the unobservable degrees of freedom of the central extension of Galileo group. Moreover, viewing the Galileian invariant quantum mechanics as a non-relativistic limit, we prove that the above-mentioned averaging procedure giving rise to Bargmann superselection rule is nothing but an effective de-coherence phenomenon due to time evolution if assuming that real measurements includes a temporal averaging procedure. It happens when the added term $Mc^2$ is taken in the due account in the Hamiltonian operator since, in the dynamical approach, the mass $M$ is an operator and cannot be trivially neglected as in classical mechanics. The presented results are quite general and rely upon the only hypothesis that the mass operator has point-wise spectrum. These results explicitly show the interplay of the period of time of the averaging procedure, the energy content of the considered states, and the minimal difference of the mass operator eigenvalues.