Loading once fired brass and necks do not need trimming, but they are not all the same length should that matter that much at all. Also for a different gun I have found my load and want to load a couple boxes but new brass has been purchased and am wondering if my gun will shoot any different since new brass has not been fireformed to my chamber. I found my load so I don't want to shoot a 100rds. jst to form and then load again.