I neck down in 10% or less increments per what Jim Leahy [James Calhoon] tells me the wildcat society journal tells him to do.
The outside of a 30-30 neck is 0.333" and 25-35 neck is 0.288".
The 30-30 FL die neck is ~0.326"
The 25-35 FL die neck is ~ 0.272"
10% less than .326" is .293"
10% less than .293" is .264"
10% bigger than .272" is .302"
So we need a neck die between .293" and .302"
Any 7mm FL die would do, but most are too long.
A 7mmBR die will work, or take a piece of 7/8-14 threaded steel rod, put it in the lathe, drill a 19/64" hole in the center [0.297"] and make a chamfer at the orifice and polish the inside of the home made neck die with sandpaper on a stick technology while the lathe is spinning.
1) Run the brass through the 30-30 die
2) Run the brass through the 7mmBR die or the 19/64" home made die
3) Run the brass through the 25-35 die
4) Trim the brass to 2.000"
5) Fire form with 5 gr Unique and 100 gr bullet. [12kpsi]
You could do all that. I have done all that.
I have also just put the 30-30 brass in a the 25-35 die, trimmed, and loaded up to 87 kpsi QL with real stiff bolt lift.
What confused me for a while as a rough 25-35 chamber. Make sure it is smooth.