Can someone tell me why you're supposed to trim before sizing.Speer#10 manual.

You don't. You trim after sizing.

thanks for that,I knew that when you size it stretcbes it out so why trim first,was in the book

I agree that the best is to trim after sizing.....but when you use a progressive reloader (Dillon) you don't want the extra step so I trim a long ways back before sizing.....if all of reloading was done with a rockchucker style press then I'd say to trim after resizing.