The existing modeling views allow with particular accuracy to estimate physical and mechanical properties of materials even at the stage of their discussion without long-term procedures of producing and carrying out tests. For example, when estimating composites’ properties, currently, “mixture rule” is widely used that allows calculating strength properties and electrical resistance of composites based on the compositions, which are determined by the components’ volume ratio. It is evident that the “mixture rule”, as all modeling approaches, has its limitations, which the authors would like to estimate using one composite with various volume ratios of the components. Using the fluid extrusion method, three Cu/Mg composite bars were produced, the copper matrix of which contained 1, 7 and 49 thin magnesium fibers. The educated estimates of strength properties and electrical resistance of the deformed composites were carried out using the “mixture rule”. The authors compared the estimated data and the experimental results. It is shown that the fullest conformity of the calculation results with the experimental data can be observed in the composites with 7 and 49 magnesium fibers. In their turn, the estimated strength properties deviate widely from the experiment in the case of practically the same volumes of components in a single-core composite. It is caused by the difference in the mechanisms of deformation of a copper matrix with the FCC lattice and the magnesium HCP-fiber. The results obtained for a composite with the 49 magnesium fibers five the idea of the initial stages of formation of new phases on the Cu/Mg interface in response to the mechanical fusion processes during the severe plastic deformation.