Try one case and see if it will pull down in one pass. If not anneal just the neck on another case not the shoulder and try it again . If it still does not work well then you will need an additional step die .
we have done this one we tried to do them in one step and it dont work. we step down to 338rcm 300rcm 7mm 6.5mm. i used the ruger compact dies to step down to 300 because of cost. we have a 7 lrm so that to 7mm and then the 6.5 ruger. i think you could get away with going from 300rcm to 6.5. as for barrel life i have 800 rounds in a 26in douglas barrel and i didnt shorten the case.