Metallic Mg insertions in rf deposited MgO barrier of magnetic tunnel junctions structures were investigated in a resistance-area (RA) range from 1 to 1000 $\Omega$$\mu$m$^2$. For the first time, investigations on Mg insertions above the MgO barrier and simultaneously on both sides of the barrier are reported. It is shown that for RA larger than 5 $\Omega$$\mu$m$^2$, a bottom Mg insertion does not improve the tunnel magnetoresistance (TMR) ratio compared to a sample with no Mg insertion. Furthermore, a top Mg insertion yields a lower TMR ratio decreasing as the Mg thickness is increased. On the other side, for RA lower than 5 $\Omega$$\mu$m$^2$, there is no significant difference between top and bottom Mg insertions indicating that in this region, the MgO crystallization occurs mainly during the annealing process. In the RA range investigated, there is no significant difference in coupling field for different insertions. In very low RA regions between 1 and 10 $\Omega$$\mu$m$^2$, an increase in TMR is observed for 0.3 nm insertions simultaneously below an above the barrier.