I have a ton of 25-06 brass that I want to use for a 6.5-06 that I am reloading for. My question is... do I need to buy a neck expander die or can I use the sizer/decapping die? This is my first attempt at resizing brass to accomodate a different caliber bullet.

You "should" be able to simply use the expander. Some brands have more of a taper on the expander than others. Lube the inside of the neck well and run it in the die.

I usually go in about half way pull back turn about 180 and go the rest of the way. Your neck might be a touch off center. It will correct on the first forming. Also the mouth might no be square. I normally square up after the first firing. Other option to square everything to the shortest side of the shortest case.

i LIKE TO USE ONCE FIRED , 25/06.. the neck is blown out to about the right dimintions.
If its sm AI chamber you will need to fire form.
I modified a set of dies to get the length I needed for my chamber.
Dave

Thanks for the advice men!!