Bryndon,

I have always had good luck blowing out brass to the chamber size on 400 Whelen plus many other calibers. I use about 10 gr of bullesye and then fill the case with corn meal. Just shoot it and the brass then fits. I then load a cast bullet long enough that it engraves and make the final case forming. I have found trying to expand the necks with expander plugs, the necks don't end up very even, the blowing out is much easier and faster and more consistent.
Dan