I believe that Armory has it right...shooting too many .38 special cartridges in the mag barrel will tend to leave a fouling ring in the chamber. If you are careful, only shoot a couple, and clean the chamber regularly, I don't see that it would hurt much. If you do shoot a whole bunch of the .38's, and then switch to the mag, you will start to notice a tight fit with the mag cartridges. On occasion, I will shoot a box or two of .38's through my Ruger GP100, followed by some Mags, and I have yet to have a problem with the cylinder, however I do clean it very regularly.