Always neck size other rifle's brass?

[zanes]

Chaps,

Interesting one here. I've always been told and read that when you're reloading cases that have not been fired in the rifle you're reloading for you must full length size. No qualifications, no "but if they chamber OK you're OK neck sizing only".

My question is- what am I missing here; the only reason I can think of for full length sizing is to prevent issues with the brass not fitting in the rifle's chamber (for a bolt action for target shooting). I have a quantity of once fired .300 Win Mag brass for my new to me rifle which was not fired in the rifle.

Happy to full length resize this time if necessary, but prefer neck sizing and trying to work out why I shouldn't.