FIELD NOTES / BRITISH SHOOTING GUIDE

Chokes & patterns

British 12-bore sizes, from cylinder to full. See how constriction changes the illustrated pattern.

01 / KNOW THE SIZES

From open to tight.

Typical traditional British 12-bore constrictions.
These are reference values, not universal specifications.

British choke sizes and typical constrictions
Choke	Thousandths of an inch	Millimetres	Character
CYLTrue cylinder	0 thou · 0.000″	0.000 mm	No constriction
SKSkeet	About 5 thou · 0.005″	About 0.127 mm	Very open; maker-dependent
ICImproved cylinder (traditional British)	About 5 thou · 0.005″	About 0.127 mm	Light constriction
¼Quarter	10 thou · 0.010″	0.254 mm	Open all-round option
½Half	20 thou · 0.020″	0.508 mm	Moderate constriction
¾Three-quarter	30 thou · 0.030″	0.762 mm	Tighter constriction
FFull	40 thou · 0.040″	1.016 mm	Tight traditional designation

Constriction is the reduction in diameter relative to the bore. Millimetres above are exact conversions of the nominal inch values, not manufacturing tolerances. [1]

WATCH THE PRINCIPLE

See the shot spread.

Select a choke, then play or scrub through the diagram.

Choke

Skeet: a very open choke, often about 0.005″ constriction. The maker’s specification takes precedence.

Travel through the diagram 0%

Slowed down and exaggerated for explanation. No distance, time, pellet energy or real pattern size is being calculated. The same number of dots is used for every choke.

02 / SEE THE DIFFERENCE

Same distance. Different distribution.

Illustrative pellet patterns, viewed head-on.
Every circle represents the same area.

More open

Pellets spread over a wider area. Useful coverage, but fewer pellets concentrated in the centre.

Moderate

A balance of spread and central density. The cartridge still has a major influence.

Tighter

More pellets grouped towards the centre. A smaller useful spread can demand greater precision.

Conceptual diagrams only. They do not predict pattern diameter, pellet count, effective range or a particular choke’s performance.

Why there is no “30-yard choke”

Patterns change with the barrel, wad, shot material, pellet size and cartridge. A choke label alone cannot establish an effective range. Test the combination at the distances you actually shoot. [1]

What does “pattern percentage” mean?

The traditional comparison counts the proportion of the cartridge’s pellets falling within a 30-inch circle at 40 yards. That is a test convention, not a recommended distance for every target or bird. [1]

PUT THE LABEL IN CONTEXT

Your pattern is the evidence.

Use an approved facility

Ask your shooting ground or coach about a suitable pattern plate and its safe-use procedure.

Use your actual cartridge

Compare patterns from the gun, choke and load you intend to use, at relevant distances.

Look beyond the centre

Check evenness and gaps over several patterns. One attractive result is not the whole picture.