It boils down to personal choice. Some say "don't use Specials in Magnum guns 'cause of the dreaded carbon ring". Some say, "go fer it". I got y first 38 Special in '69 and started reloading for is shortly after. I got my .357 Magnum in '75 and since I had a bunch of 38 brass, most of my reloads for the 357 were in 38 brass. I cleaned my gun after every shooting session. I have fired as much as 150 rounds of Specials in my Magnums before the crud became noticeable (it depends on the load, cast bullets dirtiest). After I acquired a bunch of 357 brass I started using more 357 brass than 38 brass in my 357 revolver and I even used plain old 38 Special data in my 357 Magnum brass. I can't remember any problems in my varied reloading and I still have all the guns involved...