In the 300 WSM, after the first firing, the case length usually shrinks just a little as the case diameter is expanded. That is also the reason you will most probably find that a partial resize won't work. If you're one of the very few lucky ones, you might be able to neck size, but chances are you will have to full length size.
From what I've seen and also read about, Federal brass isn't real great for reloading. I've had oversized primer pockets with once fired Federal, and it seems many others in the forums have also.