May 29th, 2008

TECH TIP: Use Rubber O-Rings with Sizing Dies for Less Run-Out

Here’s an inexpensive tip that can help you load straighter ammo, with slightly better measured concentricity (i.e. less run-out) on the case necks and bullets. Simply use a rubber O-Ring on the underside of the die locking ring. This allows the die to self-align itself (slightly) to the case that is being sized. Without the O-Ring, if the flat surface on the top of your press is not perfectly square with the thread axis, your die can end up slightly off-angle. This happens when the bottom of the locking ring butts up tight against the top of the press.

reloading die O-ring

Top prone shooter German Salazar has tried this trick and he says it works: “Go to your local hardware store and get a #17 O-Ring (that’s the designation at Ace Hardware, don’t know if its universal). Slip the O-Ring on the die and re-adjust the lock ring so that the O-Ring is slightly compressed when the die is at the correct height. Size and measure a few more cases. You will probably see a slight improvement in neck concentricity as the die can now float a bit as the case enters and leaves it. This isn’t going to be a dramatic improvement, but it’s a positive one.”

reloading die O-ring

Lee Precision makes die lock rings with built-in O-Rings. Lee’s distinctive lock ring design allows the same kind of self-alignment, which is good. However, Lee lock rings don’t clamp in place on the die threads, so they can move when you insert or remove the dies — and that can throw off your die setting. By using an O-Ring under a conventional die lock ring, you get the advantages of the Lee design, without the risk of the lock ring moving.