Just loaded some rounds. Adjusted the die as recommended. Upon seating the bullets, about 60% of the bullets were tight in the neck, the remaining bullets, while not entirely loose, could be moved sightly with finger pressure. My assumption is that the necks of the brass were not of uniform thickness. Is this a correct hypothesis? Probably would not be an issue if these cartridges were just going to the range, But using them in hunting conditions causes me pause.

Would appreciate the benefit of your experiences, thoughts, and suggestions. (If it matters: 257 Roberts, 100gr Hornady ILs, R-P brass.)

Wrong forum try in the reloading section.

Could be brass variation or different amounts of spring back. Typically I polish the mandrels down for about .002-.003" of neck tension' the factory mandrel can be .001".. You can order smaller mandrels from Lee. Might try annealing the cases as this will never hurt anything. Reaming or neck turning the brass could help but you would then have even less neck tension.

Just on the off chance you have some funky bullets check their diameter. Some of the +P Roberts brass have thick necks.

I’ve had Lee Collet dies show up with the mandrel being a little bigger than it should, producing results as you describe. Just chuck the mandrel in a drill and polish it down .001 or .002 and it should be fine. You can also buy an undersized mandrel from Lee.

I’m also not a fan of Lee’s instructions for setup with this die, and have experienced more consistent results from adjusting them per mathman’s instructions. Do a search in the reloading section and you’ll find them.

John

Mathman will be along to link you to his excellent instructions for setting the collet die up. Highly recommended. Or PM him.

Mathman directions for Lee Collet die


The first thing I recommend is to ignore the instructions supplied with the die.

The second thing I recommend is to ignore whether or not the press you're using cams over at the top of its stroke.

The die squeezes the neck onto a mandrel, so for a given neck thickness there is a finite limit to how much sizing you can achieve. This die will need adjustment to suit different thicknesses of brass.

Raise the press ram to the top of its stroke. Thread the die into the press until the bottom of the sizing collet (not the die body) just touches the shell holder. Measure the neck OD of a piece of brass.

Run the brass through the die using a full press stroke. It should take no effort since if you're set up as described the die has done no sizing. Turn the die into the press about 1/4 turn. Run the brass in again. You probably won't feel much sizing going on, but give the neck a measurement just to see. If it's still nothing, screw the die in another 1/4th and try again. You'll may start feeling a bit going on as you work the press handle, and if so you'll be able to measure a little sizing taking place.

Rinse and repeat using 1/16th turn in increments for the die. You'll feel increases in the force required for the sizing stroke. Since you're measuring the neck after each pass you'll eventually find two increments where the neck didn't get any smaller. NOW STOP TURNING THE DIE INTO THE PRESS. Remember you're squeezing the brass against a solid steel mandrel which isn't going to give, so even if the press stroke didn't feel like it took very much force the neck is as small as it's going to get.

There's a learning curve to the die, but it isn't hard.

I like to run cases through the die twice, spinning the case about 1/3 turn (rather than the 1/2 turn in the instructions) between passes. This means the parts of the neck that were under the splits in the collet fingers on the first pass will get hit on the second.