Before You Start

Two things need to be confirmed before you load your first case. Get these right once and they stay right for the entire trimming session:

First, verify you're using the correct collet for the case head diameter of the cartridge you're trimming — consult the collet selection chart in your MatchMaster documentation. Second, confirm your pilot is the right size for your caliber. If you're unsure how to identify your pilots, see the companion guide linked in the sidebar.

The Truing Process

Follow these four steps in order. The key is in step three — the slight loosening that allows the case to self-center. Don't skip it or rush it.

1. 1
   Seat the Case in the Collet

   Place the case into the handle collet, positioning the case head in the center of the collet step. Once seated, give the collet a firm twist to snug it down. The case should be held securely — not loose, but not crank-tight at this point.

2. 2
   Bring the Case Forward to the Cutter Head

   With the case snugged in the collet, advance it forward until the case mouth seats onto the pilot. You're not trimming yet — you're simply getting the case mouth engaged with the pilot so the self-truing step can happen.

3. 3
   Slightly Loosen the Collet to Let the Case Self-Center

   This is the critical step. With the case mouth on the pilot, slightly loosen the handle collet — just enough to release the grip on the case without letting it fall free. The pilot will allow the case to find its true center, correcting any minor misalignment between the case axis and the cutter axis. Give it a moment to seat naturally onto the pilot.

4. 4
   Retighten and Trim

   While maintaining light forward pressure — keeping the case mouth engaged with the pilot — retighten the handle collet to lock the case in its self-centered position. Now advance the case into the cutter blades to trim. The case is now running concentric with the cutter axis, and the result will be a clean, square case mouth.

Why This Works

Case necks aren't always perfectly concentric with the case body — especially after multiple firings. If you lock the case into the collet before it has a chance to align with the pilot, you're trimming to whatever axis the collet happened to grab, not the true axis of the case neck. That's how you get angled case mouths.

The self-truing technique solves this by letting the pilot — which fits snugly inside the case neck — physically pull the neck into alignment with the cutter before you lock anything down. You're trimming to the case neck's actual center, which is exactly where the cutter needs to be working. The result is a flat, consistent case mouth that seats bullets squarely and uniformly across your entire batch.