One of the critical parameters of the high Jc internal tin Nb3Sn strands designated for the high field magnets of the FCC project is essentially high RRR value exceeding 150. In this work Oxygen free high-purity copper with RRR>400 was used for production of internal tin Nb3Sn wires of 0,7 and 1 mm in dia with 37 sub-elements in distributed diffusion Nb-barriers. Final reactions from 25 to 100 hours were given at 665 oC to investigate the changes in RRR-values. It was shown that target for RRR-level of 150 is realized for duration time not more than 50 and 25 hours for strands with 1 and 0.7 mm in dia accordingly. It has been shown that RRR is affected by tin diffusion through weak regions of Nb-barriers. It was revealed, that irregularity shape of diffusion barriers has been the decisive factor that influenced on the time limit at the chosen temperature of final stage of reaction heat treatment. The model for RRR-degradation due to tin diffusion into copper matrix is proposed.