A USPSA Mag Gauging Guide: Pass Every Check

• A “140 mm” mag is really checked to 141.25 mm — but don’t build to the max.
• Build in margin. Target 141.000 mm or just under.
• Check every mag fully assembled, not as a pile of parts.
• Keep each basepad matched to its tube. Mark them as sets.
• If a mag is a hair long, fix the contact point — then function test it.

The rule is 141.25 mm — but don’t build to it

In USPSA, what everyone casually calls a “140 mm mag” is actually checked against a 141.25 mm limit. The current USPSA Appendix E1 magazine measurement procedure lists the relevant measurements as 141.25 mm and 171.25 mm, and names the EGW mag gauge as the primary measuring device. It also says the mag must go into the gauge without significant force, that slight pressure may be applied, and that if the fit is questioned, the Range Master’s call is final.

NROI explains the reason for the extra 1.25 mm: the USPSA gauge is intentionally longer than the nominal magazine length to allow for manufacturing tolerances in tubes and basepads.

That is the rule side of it. The practical side is where people get burned.

“It fits my gauge” ≠ “it’ll pass at the match”

This isn’t theoretical. I’ve had multiple EGW gauges certified / CMM measured, and they vary enough to matter when you’re right on the edge.

If your mag only barely presses into your personal gauge, there’s a real chance it won’t pass the gauge at chrono or equipment check. That doesn’t mean your gauge is bad. It means you built the mag with no margin.

At the absolute max, everything matters: the tube, the basepad, how it locks on, whether the feed lips are proud, whether the follower sticks up, how the mag sits in the gauge, even the exact gauge being used. That’s why I don’t treat “barely fits” as a standard.

Our standard: a conservative 141.000 mm gauge

Check every assembled mag individually with a conservative gauge. I’d use something like the 141.000 mm Henning Group SAFE CHECK gauge as the go / no-go, instead of trying to live right at 141.25 mm.

Henning lists its SAFE CHECK option as 141.00 mm — a gauge meant to check mags safely inside the official measurement rather than right at the max. They also note that mags and extensions vary, and that checking every mag before a major is the responsible move.

If it fits the 141.000 gauge cleanly, I have margin. If it only fits the 141.25 gauge with pressure and prayer, I’m not interested.

Check every mag with margin before your next major, in 141.00 mm or 171.00 mm.

Keep basepads matched to tubes

The other big mistake is swapping basepads around like they’re all interchangeable. They are not.

Tube/basepad tolerance stacking is real — a basepad that’s fine on one tube can make another tube too long. NROI has specifically warned that swapping a basepad onto a different tube can produce a mag that’s too long, and recommends gauging.

Once a tube/basepad combo passes, keep it together:

• Mark the tube. Mark the basepad. Number them as a set.
• Don’t swap parts before a major and assume it’s still legal.

Watch the grip tape, too. Grip tape stuck to the bottom of a basepad or extension counts toward the measured length — the gauge reads to the lowest point. We’ve watched a shooter get bumped to Open at a Carry Optics Nationals over exactly that.

The mag that passed on Tuesday can fail on Friday if you mix parts and never recheck.

Check the mag assembled — not as parts

A mag isn’t legal because the tube is legal, or because the basepad is legal. It’s legal because the assembled mag passes.

Check the exact configuration you’ll run at the match:

Same tube · same basepad · same follower · same spring · same retaining plate · same assembled condition.

Clean mags before a major? Recheck after reassembly. Replace springs? Recheck. Change basepads? Recheck. Mag hit concrete hard enough to make you wonder? Recheck.

If it’s a hair long, fix the contact point

Sometimes a mag is just barely long. Don’t argue with the gauge — find what’s actually contacting it and fix that part carefully.

And yes, sometimes that means: grinding wheel goes brrr on the rear feed lips. But don’t be stupid about it.

You’re not reshaping the feeding geometry or turning a reliable mag into a science project. You’re removing the minimum material from the spot causing the failure. Sneak up on it. Deburr it. Polish the sharp edge. Recheck. Then function test before you trust it.

A mag that gauges but doesn’t run is not a win.

What to do if your mag fails the gauge at a match

The official procedure is USPSA Competition Rules, Appendix E1. The mag goes into the EGW gauge and must seat without significant force — the rules allow only slight pressure (light finger pressure, not a crush grip). If it truly doesn’t fit, no amount of pressure will make it.

First, know the stakes. If your mags are over length at chrono, you get bumped to Open in Carry Optics, Limited, Limited Optics, and Limited-10 — or you shoot for no score if you’re already in Open. That’s the whole match. So if a mag reads borderline at check, work the problem before you accept that result:

• Ask for that slight pressure. Feed lips can be a touch proud, followers can stick up, and a basepad can sit slightly loose — a little grip can seat it legally.
• Disassemble, clean, and reassemble it correctly — under the RO’s supervision — then re-gauge. Do it in front of the Range Officer (who can examine your equipment at any time), and reassemble with the same parts you’ll actually compete with — not a different basepad or tube swapped in to beat the gauge. Grit under the follower or basepad, a basepad seated crooked or on backwards, or a basepad swapped onto a different tube can all add length. A clean tube and correct assembly often bring a borderline mag back into spec.
• Ask the Range Master. Appendix E1 also provides a ruler / tape-measure method, intended mainly for basepads that aren’t compatible with the gauge. Either way, the Range Master’s call is final.

Better yet, get ahead of it: when you can, ask the chrono or equipment-check staff to run your mags on the match’s own gauge before you shoot. Samples vary, and the only gauge that counts is the one at the match — any mag you use can be checked at any time, so confirm on theirs early.

None of this is a loophole for running an over-length mag — it’s how you recover a mag that’s actually legal but reads borderline. Build with margin and you won’t need any of it.

✅ The major-match rule: no surprises

• ✓  Check every assembled mag in a conservative 141.000 gauge.
• ✓  Keep passing basepad/tube combos together; mark them as sets.
• ✓  Don’t swap parts around casually.
• ✓  Recheck after cleaning or changing parts.
• ✓  Fix the actual contact point if a mag is slightly long.
• ✓  When you can, gauge on the match’s own gauge at check-in — theirs is the only one that counts.
• ✓  Function test anything you modified.

The goal isn’t winning an internet argument over whether 141.24 mm is technically legal. The goal is to walk up to chrono, drop your mags in the gauge, and have absolutely nothing interesting happen.

The worst time to learn about tolerance stacking is at equipment check, with a mag that “always fit at home.”

Bottom line

USPSA gives you a 141.25 mm limit for 140 mm divisions — but that doesn’t mean you should build to barely survive it. Build in margin. For me that means treating 141.000 mm or just under as the real-world target.

Check every mag as an assembled unit. Keep basepads matched to tubes. Use a conservative gauge. If a mag is a hair long, correct the contact point and function test it.

A couple thousandths of theoretical magazine length isn’t worth getting bumped, shooting for no score, or spending the rest of the match mad at a gauge.