expander ball diamter question

[Andrew Mason]

so, i am going to have some lathe work done on some 8mm mauser dies,
i am having the expander die turned down to be the appropriate diameter for
the use of .318 diameter bullets.
what diameter should the neck expander be?
.318? .317? .316?

i will probably shoot it with j-word bullets
but, it is very possible that i will shoot some
cast boolits with it.

[nhrifle]

Usually .001" or .002" less than bullet diameter gives enough neck tension to hold the projectile and maximize brass life. What I would do is to turn the expander to bullet diameter, then polish it to the size you want so it is nice and smooth. A smooth expander makes a huge difference in resizing.

[geargnasher]

You'll probably want two of them if you shoot cast and jacketed. A lot of it depends on how the brass behaves, generally in bottlenecked stuff that runs about .013-.014" in neck thickness and is designed for more than 40K psi, .0015" interference fit works best, and that's after at least a half-thousandth case springback, so I like about .001" under boolit diameter. Case springback will be much more if you're using factory sizing dies that crush that neck way too small and have to be stretched out again. It's all a balancing act, and with custom expanders it's often a good idea to pair them with a custom sizing die, Lee Collet neck sizing die, or bushing-type neck sizer. You can also hone out the neck on your current sizing die with wet Emery paper, a dowel rod, and drill press.

Gear

[Andrew Mason]

the die i am using is a RCBS FL sizing die
so, i will shoot for .317, and then polish it.