Written by Sierra Ballistic Technician Rich Machholz
I have had this experience personally and stumbled across one solution quite accidentally. Of course there could be several things causing this, so we can explore this one by one.
If your neck is .001″ out before seating a bullet that will be amplified if measured on the bullet after seating.
The solution may be as simple as polishing the upper edge of the expander ball. Both the seating die AND the sizing die need to be squared to a dedicated shellholder in a dedicated press.
This is not normally an everyday chore, but check it every time by raising the shell holder to the bottom of the die to a light touch and watch to be sure that more than just one side touches. A light behind the press will help and if just one side does touch, resquare the dies.
If you find a true straight loaded cartridge use it to square the seating die as a quick fix by backing the seating die off and running the ram to the top of the stroke. Now back your seating stem off to make sure you don’t disturb the original seating depth and screw the die body down until it contacts the crimping ledge in the seating die and back off a half turn.
Set the seating stem snugly against the bullet and tighten the lock ring. Next tighten up and set the lock ring on the die body and you now have a quick fix.
A GOOD and UNIFORM inside champ here can solve a lot of seating problems, concentricity or lack thereof being one of them.
You can use the deep hole provision of your calipers to check the height of your shellholder as mounted in the ram and the ram itself for squareness.
Or … you can invest in a Forster Co-Ax press and benchrest dies and kiss concentricity issues good bye. Or find a press that IS square.
Fighting concentricity is very frustrating and to think we didn’t even worry about it 20 years ago, maybe even less than that, but that is also the reason today’s rifles are so outstandingly accurate.
I hope you find the cure to your problem and good luck.