Are you using a neck sizer or full length resizing die? It will make a big difference when chambering for different rifles. Neck sizing should only be used when firing the same cases from the same rifle and the plus side to neck sizing is greater accuracy since the case perfectly fits your rifle's chamber. With a full length sizer die, the brass will be worked down to the web to the SAAMI chamber specs which should work on all rifles chambered for that round.
To answer your second inquiry, no, the powder should not expand the case unless you use an immensely compressed load and even then the stretch would be negligible - usually less than .005" but this is with a load that the powder would be literally falling out of the case before bullet seating. However, the depth that you seat your bullet at will affect chambering dependent on the OAL and the bullet profile. With some longer bullets with a steeper angle, ogive (i.e. spitzer rounds), you can get away with longer OALs and still get the round to chamber properly since the bullet won't engage the rifling as soon. With the flatter bullets like soft points, that is not the case.
Hope this helps. Welcome to the grand science experiment with reloading!
Damian