I put the 30-40 brass through a 6.5 Krag sizer and shove the shoulder back so it just chambers. I can't get the 30-40 brass into the 25 Krag Ackley sizer or the shoulder will collapse. The necks of sizers always being extra small, the next size up, 6.5 is small enough for neck tension on a 257 bullet.
I drilled a hole through 3/8" Aluminum and countersunk it until the brass neck can poke through and out the other side by 0.015".
I filed the brass mouth flush with a fine file with a rabbit symbol.
I turn the brass while dragging the edge of the neck on the file, like sharpening a drafting pencil.
I run the brass through a 25-06 Lee collet neck die. That only acts on the top third of the neck, but does almost nothing, except to the inner burrs from filing. Chamfer and prime.