# How shot gauges work: plug scoring on ISSF targets explained

> A shot gauge is a machined plug placed into a doubtful bullet hole to decide whether it touches the higher scoring ring. ISSF rules fix the gauge diameter for every discipline, restrict insertion to jury members, and allow only one gauge per hole.

- Canonical URL: https://target-log.net/blog/how-shot-gauges-work-plug-scoring-explained/
- Published: 2026-09-05 (updated 2026-09-05)
- Category: Scoring rules
- Tags: scoring, intermediate
- Read time: ~8 min

## Key takeaways
- Shots score the higher ring when the hole, or the gauge in it, touches any part of the outside edge of that ring.
- 10m air rifle uses a 4.5mm inward gauge, a 5.5mm outward gauge, and an 18.0mm gauge for inner tens; 10m air pistol uses an 11.5mm outward gauge.
- Only a jury member may insert a gauge, once per hole, and the use must be marked on the target with initials.

Every club has a box of them somewhere near the scoring desk: small metal discs with a spindle in the middle, worn from years of use. Shooters call them plugs, gauges, or scoring gauges depending on where they trained, and they exist for one narrow job: settling the shots nobody can agree on by eye. A hole that clearly sits inside the nine needs no help. A hole that kisses the line between nine and eight needs a shot gauge, and the whole process is governed by stricter rules than most beginners expect.

This guide covers the gauges themselves, the edge-to-edge principle that decides every borderline shot, and the procedures from the ISSF Rules for Paper Target Scoring. If you have shot on [electronic targets](/blog/electronic-targets-vs-paper-how-issf-scoring-works/) you have already seen the alternative: sensors that never argue. On paper, the argument ends with a plug.

## What is a shot gauge?

A shot gauge is a precisely machined plug inserted into a bullet hole to determine whether that hole touches a higher scoring ring. That is all it does. It does not measure the shot's distance from centre, it does not print decimals, and it cannot overturn an unambiguous hole. When a scorer plugs a doubtful shot, they read where the gauge's measuring edge falls relative to the ring line and award the value accordingly.

Three gauge families cover the ISSF paper disciplines:

| Gauge | Measuring edge | Used for |
|---|---|---|
| 4.5mm inward gauge | 4.50mm (+0.05/−0.00) | 10m air rifle rings 1 and 2, and [inner tens](/blog/how-issf-tiebreaking-works-countback-inner-tens-and-ranking/) |
| 5.5mm outward gauge | 5.50mm (+0.00/−0.05) | 10m air rifle rings 3 to 10 |
| 11.5mm outward gauge | 11.50mm (+0.00/−0.05) | 10m air pistol rings 2 to 10 |

An 18.0mm outward gauge also exists for scoring air rifle inner tens, and 25m pistol events with tipping targets bring a skid gauge for elongated holes. Tolerances run to five-hundredths of a millimetre because the gauge is a legal instrument, not a club convenience. That is also why gauges must be inserted with the target lying horizontal: gravity can pull an inserted plug off-centre and distort the reading.

## How does the edge-to-edge rule decide a score?

A shot hole scores the value of the highest ring it touches, and a hole touches a ring when any part of the hole breaks the outside edge of that ring's line. Rule 5.2.1 of the paper scoring annex states it directly: if any part of a higher-value scoring ring is touched by the bullet hole, the shot receives the higher value, determined by whether the hole or a plug gauge inserted in it touches the outside edge of the ring.

The direction of the measurement is what the gauge enforces:

- An **inward gauge** has a measuring edge of the true bullet-hole diameter. Insert it, and if the gauge's edge reaches out to the ring line, the shot counts the higher value. It proves the hole itself touched the line.
- An **outward gauge** has a flange larger than the hole, sized so the outside of the flange sits one ring-width plus one hole-diameter from the spindle. Seat the spindle in the hole; if the flange's outer edge stays inside the higher ring, the hole must have touched that ring's line.

The outward gauge is easier to read because you compare the flange edge against a zone rather than a hairline, which is why ISSF scoring uses it for most doubtful holes. The classic illustration from the rules: with the air rifle outward gauge in place, its flange edge lying within the 7 ring scores a 9, while the same edge crossing over the 7 ring line into the 6 zone scores an 8.

## Who is allowed to insert a scoring gauge?

A jury member, once, with the act recorded on the target. Rule 5.2.5 says the gauge may be inserted only once in any bullet hole and only by a jury member, and that gauged shots must be marked on the target with the official's initials and the result.

The once-only rule protects the paper. Every insertion enlarges or tears the hole slightly, so a second plug could effectively redraw the shot. A gauge used at a match is marked because the target is evidence: if a result gets protested later, officials need to see exactly which holes were gauged, by whom, and what was awarded.

At club level this formality loosens, since few clubs field a jury. The scorer at the desk plugs the doubtful holes, and shooters accept it. Just know that at a sanctioned competition, grabbing the gauge yourself and stabbing it into your target is a rules violation, not a shortcut.

## What happens with torn, overlapping, or skidded holes?

Real targets produce problem holes that no plug can seat into cleanly. The rules handle each case.

When another bullet hole sits so close that the gauge cannot seat, or the hole is badly torn, scoring switches to an overlay: a flat transparent sheet with an engraved ring of the appropriate size, laid over the target to reconstruct the true hole position and read it against the ring. Nothing gets inserted, so nothing gets damaged.

On 25m pistol events with tipping targets, a shot can leave a horizontally elongated hole where the target was turning. That is what the skid gauge is for, a transparent plate with two parallel engraved lines: when the inside edge of the lines touches a scoring ring, the shot takes the higher value.

A different edge case that beginners often worry about: a hole in the black that passes clean through the nine ring into paper beyond the target face still scores by where its edge falls, and [how ISSF scoring works](/blog/how-issf-scoring-works/) overall follows the same touch principle everywhere, from gauges to electronic decimal scoring. One principle, several tools.

## How should you use a shot gauge on your own training targets?

Nothing in the rules stops you from gauging your own practice cards, and it is a good habit. A consistent method:

1. Score all unambiguous holes first, by eye, and mark the doubtful ones with a pencil dot rather than a value.
2. Lay the target flat on the scoring desk. Never gauge a target held against the light at an angle.
3. Insert the correct gauge for your discipline gently, spindle first, without twisting it into the paper.
4. Read the measuring edge against the higher ring's outside line and write the value in the corner of the relevant shot.
5. Expect the paper to show the insertion afterwards, so keep gauging for competition-relevant cards and doubtful holes only.

If you photograph your targets with TargetLog instead, the app measures hole positions against the same ring geometry and applies the same touch rule, which settles most doubtful shots before anyone reaches for a plug. The [download page](/#download) has the app; the [scoring with your phone guide](/blog/how-to-score-your-issf-targets-with-your-phone/) walks through the workflow.

## Should you buy your own gauges?

For a club shooter moving into scored competitions, yes, eventually. A personal 4.5mm and 5.5mm pair for air rifle, or the 11.5mm outward gauge for air pistol, costs little compared with a tin of match pellets and removes the wait at the shared scoring desk. Two cautions before you order: buy from a recognised manufacturer, because the tolerances are the whole point of the tool, and check which rings your discipline's rules assign to inward versus outward gauges so you use the right one. A gauge that reads the wrong edge produces confident wrong answers, which is worse than an honest squint.

Treat gauging as part of scoring literacy rather than a niche ritual. The shooter who understands why an outward flange decides their nine, and who can read a gauged target at a protest, walks into championships knowing exactly where their points come from.

## FAQ

More shot gauge questions that come up at club level.

**Do electronic targets use gauges at all?** No. Acoustic and optical sensor systems compute ring values from the measured impact position, and decimals in finals come from that measurement. Protests at electronic-target matches go through the range officer and jury instead, as covered in the [electronic targets explainer](/blog/electronic-targets-vs-paper-how-issf-scoring-works/).

**Why are there inward and outward gauges if they answer the same question?** Readability. An inward gauge asks the scorer to judge whether a machined edge touches a printed hairline, hard at the edge of human vision. An outward gauge converts the same question into whether a wide flange edge stays inside a zone, which is easier to judge consistently across long scoring sessions.

**Does a larger bullet diameter change the gauge?** Yes, that is the entire design logic. Gauge sizes follow from bullet diameter plus scoring-ring geometry, which is why .22 LR disciplines carry different gauges than 4.5mm air gun events. The edge-to-edge rule stays identical everywhere.

**Can a plug gauge turn an 8 into a 9?** It can confirm that a hole touching the nine's ring line scores nine, where the naked eye called it an eight. It cannot move a hole or add value to a shot that genuinely missed the line. The gauge reveals what the hole already did.

## FAQ
**Q: Can a shooter ask for a gauge on their own target?**
A: Yes, through a protest. The jury then decides whether to gauge the hole, and if they do, only a jury member may insert the gauge and only once. A denied protest at an electronic-target match costs two points.

**Q: What if two bullet holes overlap so the gauge cannot seat properly?**
A: ISSF rules switch to a flat transparent overlay with an engraved ring of the correct size, which reconstructs the true hole position without inserting anything. Torn holes get the same treatment.

**Q: Do I need my own scoring gauge as a club shooter?**
A: It helps but is not required. Most clubs keep a set at the scoring desk. A personal gauge is worth buying once you shoot scored competitions regularly, because it lets you check doubtful holes on your own training cards.

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