RCBS's MatchMaster Precision Case Trimmer, proudly partnering with Henderson Precision, is the best way to speed up your case prep and get you back to reloading. If you're using your trimmer to trim, chamfer and deburr more than a couple of cartridges, you've probably had the question — "Which collet do I need for XYZ cartridge?" and "Why is my cartridge listed as compatible with two different collets?"

The MatchMaster System: Collets and Rim Diameter

Most case trimmers hold brass one of two ways: by the shoulder, or by the rim with a shell holder that's basically the same part you use in your press. If that's what you're used to, the RCBS MatchMaster case trimmer collet chart looks like nonsense.

The MatchMaster Precision Case Trimmer works differently. It grips the case by the rim, in a Brown & Sharpe style collet, and it references trim length from the case head forward. Not from the shoulder. Not from the neck.

Because the collet closes on the rim, rim diameter decides your collet — not case length or body diameter. A stubby .223 and a long .30-06 share a collet, because their rims step neatly into the same seat. A .30-30 doesn't share a collet with either one, even though it lives in between them on the shelf.

What the Three Numbers on the MatchMaster Collet Mean

Look at a MatchMaster collet listing and you'll see something like ".379, .473, .532" next to collet #1.

Each MatchMaster collet is a stepped collet. Inside it there are separate seats, each machined to one common case rim diameter. Those numbers are distinct steps, not a span or a range from smallest to largest.

Collet #1, included with your trimmer, covers a wide range of cartridges. The .379" step matches the small rimless head, cartridges like the .223 Remington. The .473" step is the "standard" or .30-06 head size, shared by the .308 Winchester, .30-06 Springfield, .243 Winchester, .270 Winchester, 6.5 Creedmoor and 7mm-08 Remington. The .532" step is the H&H belted magnum family, so .300 Win Mag and 7mm Rem Mag land there.

Collet #3 covers the middle ground plus some of the modern beltless magnums — a .440" step for the .441" head cartridges like the 6mm ARC and 6mm PPC, and a .545" step for the WSM, RUM, SAUM, PRC and Nosler family cases. If you're going to buy one additional collet, this would likely be it.

Collet #2 leans rimmed, with the .30-30 Winchester and 7.5x55 Swiss.

Collet #4 goes big: a .605" step for the .300 Norma, .338 Norma and .338 Lapua Magnum.

MatchMaster Collet Cartridge Chart

If your cartridge is on this chart, use the collet it’s listed under. If it isn’t, measure your rim and find a listed cartridge that shares it.

Collet #1 — .379, .473, .532

Ships with the trimmer. The .379" step is the small rimless head. The .473" step is the standard or .30-06 head. The .532" step is the H&H belted magnum family.

Collet #2 — .350, .418, .506

Leans rimmed.

Collet #3 — .356, .440, .545

The .440" step is the .441" head cartridges. The .545" step is the WSM, RUM, SAUM, PRC and Nosler family. The one to buy if you're only buying one more.

Collet #4 — .310, .605

The .605" step is the big Norma and Lapua magnums.

Why Some Cartridges Can Use Two Different Collets

There are some cases that could plausibly go two ways, and this is where the geometry gets interesting.

Take the .300 Winchester Magnum and the .300 PRC. Both have a rim measuring right around .532". Yet the Win Mag lives on collet #1's .532" step and the PRC lives on collet #3's .545" step. That looks like a mistake until you remember that the collet has to clear the case, not just grip it. Beltless and rebated cases like the PRC, the WSM and the RUM carry more body diameter just ahead of the rim than a belted magnum does. The larger step gives that body somewhere to go.

There are also cartridges whose rims land a few thousandths off one step and a few thousandths off the next. It's another way you end up looking at two plausible answers.

In cases like these, either collet may physically hold the case. Collets are built with a little tolerance for rim variation, because rim diameter is one of the less tightly controlled dimensions in brass manufacturing. The steps have to be forgiving or nothing would fit twice in a row.

Or you may be reading two different charts

There's a simpler explanation worth ruling out first. MatchMaster collets also fit Henderson trimmers, so charts for both machines are out there and they don't have to number things the same way. If you searched your cartridge and landed on a chart that wasn't ours, the same case can show up under two different collet numbers for no geometric reason at all. Confirm the chart is the RCBS MatchMaster chart before you order.

Which MatchMaster Collet Should You Buy?

If your cartridge is listed on the chart, use the listed collet.

If your case is between two steps, pick the step that matches your rim diameter most closely. The collet closes onto the rim, so the nearest step gives you the most even grip all the way around. Remember the trimmer references length from the case head — so if a case doesn't seat squarely and fully against the right internal step, that error doesn't disappear. It shows up in your trim length. A case that's cocked a few ten-thousandths in the collet is a case that comes out the wrong length.

And if your cartridge isn't on the chart at all — which happens, especially with wildcats and newer designs — measure your rim with calipers and find a listed cartridge that shares it. If the collet holds a .308 Winchester, it'll hold anything else with a .473" rim. If in doubt, contact RCBS.

See It at the Bench

Two videos worth your time.

Summary

Rim diameter picks your collet. Each MatchMaster collet holds two or three separate steps, not a range.

And if you saw your cartridge under two numbers, check the geometry first, then check that both charts were for your trimmer.