Trimming brass is the reloading chore nobody looks forward to. You size, you measure, you trim, you chamfer, you deburr, and then you do it again after the next firing because the case grew another thousandth or two. Multiply that across five hundred pieces of .223 and you understand why handloaders ask whether there's a better way.
One solution to minimize trimming is RCBS's family of X-Dies. Trim the case once, set the die up correctly, and you're done trimming that lot of brass for a long run of firings. Here's how our X-Dies work, and how to set them up.
How RCBS X-Dies Work
Inside an X-Die is a proprietary mandrel — the black adjustable rod running down through the top of the die body, carrying the expander ball and decapping pin. In a standard full-length sizer, nothing stops the case mouth as brass flows forward during resizing. In an X-Die, the case mouth runs into the mandrel stop.
That contact is what limits growth. Because the tolerance between the mandrel and the die neck wall is extremely close, the neck wall does not thicken as the case length is pushed back. Our own instruction manual puts it plainly:
"This die is not intended to shorten the length of the case, rather, as the case grows, the mandrel will reduce the normal rate of growth."
RCBS X-Die Instruction Manual, "About the X Die" (PDF)Two things follow from that, and both surprise people the first time out.
First, your cases will grow a few thousandths of an inch initially, and will then stabilize below the maximum case length. That initial growth isn't a failure. It's each case settling until it seats firmly against the mandrel stop.
Second, this is a full-length sizing die with an extra job. You can run it with or without the X mandrel feature. Partial neck sizing works too.
For scale on what "stabilized" case growth looks like, Ultimate Reloader tested a .223 case trimmed to 1.740" and reported it didn't grow another 0.001" until the eighth firing when using our X-Die. In a later round-up they measured 0.0045" of case length growth after 20 firings.
Which one do you need? If you're just starting a new caliber, order the X-Die Full-Length Die Set — it includes the X-Die sizer and a standard seater die. If you already own a set of dies for that caliber, you only need the X-Die Full-Length Sizer. There's also an X-Die Small Base Die Set, which the manual specifies for automatic, semi-automatic, pump, slide and some lever-action rifles.
Die Prep
As with all new dies from RCBS's factory, we recommend an initial clean to remove preservative chemicals and any leftover oil from the manufacturing process. X-Dies have a small vent hole that lets trapped air escape during sizing. On a brand-new die, that vent hole can be plugged with factory polishing compound, and that material will scratch and dent your cases.
To prepare your X-Die:
- Unscrew the mandrel from the die body
- Push a straightened paper clip, a needle or something similar into the vent hole to drive the material into the die body
- Clean the inside of the die with a solvent-saturated patch
- Reassemble
One-Time Case Prep
Every case in the lot has to start at the same length. This is a one-time job for any and all cases you plan to run in that die.
- Full-length resize every case. Use an X-Die Full-Length Sizer Die set up for plain full-length work per the "Preparing Your Cases" section of the manual, or just use a regular full-length sizer. Either is fine. If you're using the X-Die for this pass, screw the mandrel in until you feel hard resistance and then back it out one to two turns before tightening the lock nut. That backs the X feature off so you're sizing without it — which is what you want, because the cases aren't trimmed yet.
- Trim to .020" under maximum case length. This gap is the room the case needs to grow into before it settles against the mandrel.
- Chamfer and deburr the case mouth as you normally would. A hand chamfer/deburr tool, the RCBS MatchMaster Precision Case Trimmer or another 3-Way Cutter does the job.
- Clean off any lubrication before you move on.
Whether you're starting with virgin factory brass or once-fired brass for your X-Dies, this is the same. Size, trim to .020" under max, prep the mouth.
Set Up the X-Die
- Back the mandrel out until the decapping pin extends roughly 1/16" to 1/8" below the base of the die.
- Screw the die into the press until it touches the shell holder with the ram at the top of the stroke. Then lower the ram and turn the die down another 1/8 to 1/4 turn so the press cams over center. That takes the play out of the press linkage. Make every effort possible to duplicate the same amount of cam force each time the die is used. On an X-Die, consistent cam force is what keeps the mandrel in the same relationship to the case mouth every stroke.
- Tighten the large lock nut.
- Run your prepped case fully into the die, then screw the mandrel down until you feel hard resistance. When you're set correctly, roughly four turns of thread will still be showing on the mandrel above the lock nut. Tighten the mandrel lock nut.
- Test. Reinsert a case and look for a bulge between the shoulder and the case body. Then chamber the first few cases at the start of the session. If they won't chamber because of a bulged shoulder, the mandrel is too far down and you need to reset it.
From there you size cases exactly as you would with any full-length die.
Troubleshooting
- Cases still growing. The mandrel isn't far enough in, which usually traces straight back to setting it before trimming instead of after.
- Buckled cases. The mandrel is adjusted too far down. This is the opposite failure from cases that keep growing, and it's why the manual has you check the first few cases before running the lot.
- Inconsistent results batch to batch. Look at your die-to-shell-holder adjustment. Duplicating cam force each time is what makes the mandrel position repeatable.
X-Die FAQ
Do I need to anneal for the X-Die to work?
No. Annealing isn't part of X-Die operation, and the die doesn't reduce or undo work hardening — it's a mechanical stop that limits how far the case can lengthen, and it makes no metallurgical claim. As always, firing and resizing brass work-hardens it and you'll want to use your normal process for annealing your cases.
Can I use one X-Die across two lots or batches of brass at different lengths?
Not well. The X-Die mandrel is set to one trimmed length. Keep a trimmed lot together and treat it as a batch. The manual is direct about this: maintain the setup throughout the entire life of the cases in that trimmed lot.
Watch It Get Set Up
Two videos worth your time.
Stop Trimming Brass: RCBS X-Dies Explained
Ours walks through the die and what it's doing.
Watch on YouTubeNever Trim Again: The RCBS X-Die
Ultimate Reloader filmed a full setup and a multi-firing test in .223, which is useful for seeing the mandrel adjustment and the case-length measurements over repeated cycles. Gavin also wrote the test up with his load details and measurements.
Watch on YouTube
Trim once. Then get back to loading.
X-Dies are available as full sets, sizers only, and small base.
Always follow safe reloading practices. Consult current published load data and your equipment's instruction manuals before beginning any reloading operation.
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