For Better Bullet Seating — Try Lapping Inside of Seating Stems By:

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June 15th, 2024

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Erik Cortina bullet seating stem polish lap lathe

Here’s a simple task you can do that will give your seater die a more perfect fit to your match bullets. You can lap the inside of the seater stem so that it matches the exact profile of the bullet. This spreads out the seating force over a larger area of the bullet jacket. That allows smoother, more consistent seating, without putting dents, creases, or sharp rings in your bullets.

Erik Cortina bullet seating stem polish lap lathe

This process is demonstrated here by our friend Erik Cortina of Team Lapua-Brux-Borden. Erik, one of the nation’s top F-Class shooters and a skilled machinist, explains: “Here I’m lapping my new seater die stem with lapping compound. I chuck up a bullet in the lathe and lap the inside of the seating stem. I put lapping compound on the bullet and also in the stem. You can do the same with a hand drill and bore paste. You can see in the piture below how much contact area the stem has on the bullet after being lapped. This bullet is a Berger 7mm 180-grain Hybrid. ”

Erik Cortina bullet seating stem polish lap lathe

Q1: Is Lapping Seating Stems really necessary?

It can be helpful but it’s not necessary to make your seating stem an exact match to a bullet, particularly if you’re loading hunting or varmint rounds. But it is helpful to do some mild internal stem polishing. This should eliminate any ring (or dent) that forms on the bullet jacket during seating.

bullet seating stem lapping Erik Cortina
Photo credit Sierra Bullets.

Sharp edges on a seating stem can cause a ring to be pressed into the bullet jacket — especially with compressed loads that resist downward bullet movement.

Q2: Is there any down-side to the process?

Not really. However, if you shoot many different bullet types for a particular cartridge, you may not want to conform the stem aggressively to one particular bullet design. Lightly lap the inside of the stem to remove burrs/sharp edges but leave it at that. A light lap will prevent a ring forming when seating bullets.

bullet seating stem lapping Erik Cortina
Photo credit Sierra Bullets.

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Tags: Bullet Seating, Erik Cortina, Lapping Seating Stem, Polish Die Stem, Reloading, Seating Die, Seating Stem