Same calibre, different fit: barrels, calibre and head size

Two tins can both say ".177" and still not be quite the same pellet. Two barrels can both say ".177" and not be quite the same barrel either. The difference lives in hundredths of a millimetre — too small to see, large enough to matter once you write groups down.

What ".177" and "4.5 mm" actually mean

".177" is a calibre name in inches. 0.177 inches works out at about 4.50 mm, which is why the same family is also sold as "4.5 mm" (0.177 × 25.4 = 4.4958). The name labels a family. It is not a promise that every barrel or pellet in that family measures exactly 4.50 mm.

A rifled barrel has two diameters, not one. The bore (across the tops of the lands) is the smaller. The groove diameter (across the bottoms of the grooves) is the larger. One major barrel-blank maker publishes both for a standard .177 blank: about 4.46–4.47 mm on the lands and about 4.62 mm in the grooves (Lothar Walther blank sheet gives 0.176 in; Huma-Air's listing of the same blanks gives 4.46 / 4.62 mm; both opened 05/11/2026). Commercial pellet heads sold around 4.50–4.53 mm therefore sit between those two figures. They are not "matching the bore" in the everyday sense; they sit in the window the rifling leaves them.

That is the first thing most people are never told.

Head and skirt: two jobs, one pellet

A diabolo pellet is not a miniature bullet. The head is the solid nose that should meet the lands cleanly — firm contact to keep the pellet straight, without being forced so hard that you spend the shot fighting friction (Gaylord). The skirt is the thinner rear cup. When the air column hits it, the skirt can flare a little, seal the bore and take a bite of the rifling. That division of labour is why a soft, waisted pellet can still work in barrels that are not identical. How much engraving on the head is "enough"? There is no honest answer on paper. You find out by shooting (Gaylord).

Ullr Precision will print the head size on every tin, and we record skirt size as part of our own checks. We will not ask you to trust a feeling in your fingers.

Barrels vary. So do tins. So does "choke"

Barrels are machined and finished to tolerances. One blank responds a little differently from the next when the rifling is formed, so finished sizes wander by tiny amounts (Gaylord). Age and wear play a part too. How a particular barrel and pellet get on is "rarely predictable on paper" (Airgun World).

Many pellet barrels also have a choke: a short, deliberate tightening of the bore near the muzzle. Its job, as pellet makers have described it, is to size the pellet just before exit so the last contact is consistent (Gaylord). Push a pellet through from the breech and you can often feel a firmer spot in the last inch or two. Blank makers sell choked and unchoked tubes. Whether a choke helps your barrel is still argued: a uniform barrel may not need one; an uneven one may be helped by a consistent exit. The referee is the target.

Pellets vary inside the tin as well. Writers who grade tins with a go/no-go head gauge keep finding more than one measured head size in a single tin — and sometimes a labelled size that does not match the gauge (Morton; Gaylord; Hard Air Magazine's head-diameter analysis). That is not a scandal. It is close measurement. It is also why the number belongs on the tin, and why the label is a starting point, not a religion.

Why one head size can group better — in that barrel, with that pellet

The general idea is simple. The head should fit that rifling well: not so loose it wanders, not so tight it is forced (Airgun World; Gaylord). Because barrels differ, the head size that fits one well may not fit the next.

A Shooting UK test made the subtler point. One .177 airgun, several pellet types, heads graded into 4.50–4.53 mm, five-shot groups at 30 yards. Some types tightened at 4.51 mm; another looked happiest at 4.52 mm. Same barrel, different shapes, different preferred heads (Morton). So "my barrel likes 4.52" is incomplete until you name the pellet. Treat the test with respect and caution: one airgun, one distance, mostly one group per combination. Useful direction. Not a law.

A fair way to find out (the proof card)

One pretty group proves luck. Group size wanders around an average the way muzzle velocity does. A long-range ballistics writer puts a practical floor under that: about five groups of five before you sketch what a combination can do, and never cherry-pick the smallest (Litz, via Ultimate Reloader). Extreme spread — the usual centre-to-centre measure — only looks at the two widest shots. Mean radius (average distance of every shot from the group centre) uses the whole group and settles faster as you shoot more (GroupLab). You do not need software for a first pass. You do need more than one target.

A sensible sequence:

1. Small samples of two or three head sizes of the same pellet.
2. One change at a time — same airgun, same day, same distance, same rest.
3. Several groups of each (aim for five of five if you can spare the pellets).
4. Measure the same way every time, on a target printed at true size.
5. Write head size, lot code, date, distance and group size on a proof card. Memory is a generous judge.
6. Repeat on another day before you decide.

None of this is a rule. It is a way of letting your barrel tell you what it likes, with notes you will still believe next month.

What Ullr Precision will do

Every Ullr Precision tin will have the head size printed on it — including 4.51, 4.52 and 4.53 where we offer them — so you know what you are testing and can buy the same again. Our pellets will be tested by Ullr Precision, and the results published as measured, good or bad.

And a tool for the other half of the question

Knowing the pellet's head size is half the picture. The other half is the barrel.

Barrel-measuring tool: Coming.

Ullr Precision products are for adults 18+. Initially delivering to Great Britain.

Sources

• Lothar Walther .177 airgun blank NO CHOKE (bore 0.176″ / groove 0.182″) (opened 05/11/2026)
• Huma-Air, LW standard blanks (.177 bore 4.46 mm / groove 4.62 mm) (opened 05/11/2026)
• Tom Gaylord / Pyramyd AIR, 'What do pellet head sizes mean?' (opened 05/11/2026)
• Tom Gaylord / Pyramyd AIR, 'What about choked barrels?' (opened 05/11/2026)
• Mike Morton / Shooting UK, head-size test (opened 05/11/2026)
• Shooting UK, ammo and barrel compatibility (opened 05/11/2026)
• Airgun World, 'How to choose the right pellet for your airgun' (opened 05/11/2026)
• Hard Air Magazine, Archer head-diameter analysis (opened 05/11/2026)
• Ultimate Reloader / Bryan Litz on group variation (opened 05/11/2026)
• GroupLab, mean radius vs extreme spread (opened 05/11/2026)
• Ballistipedia, Range Statistics (opened 05/11/2026)
• Sacramento Valley Airgun Club, pellet sizing (mechanism background only) (opened 05/11/2026)
• .177 in ≈ 4.50 mm: general knowledge (0.177 × 25.4 = 4.4958)

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