It never hurts to go longer. I use the longer mags for my 338 LM as well. Especially if you intend to shoot heavy/long bullets. You can always shorten the rounds up. My 300 WM is a factory barrel and I have to run 200 ELDX .030" off to get in the mag. Pretty sure the heavier eld's are made to run long and this would benefit having longer mags. Allows more powder capacity as well if you don't have so much bullet taking up space