Re: making 260 brass
I am doing the same as you are with .260 Rem, but with Sinclair expander thing. I am getting the same results as you. However I am not sure that the bushings go all the way down the neck to the shoulder anyway. I had a 6.5 x 55, with Lapua brass the bushing on the Redding dies would not adjust so that it went all the way down to the shoulder / neck junction (about 1/8th inch short).
In short it may be to do with the die design rather than the case, but my experince with Redding bushing dies is limited to two calibers only.