Thought I'd summarize this, as it's something I always tend to forget and maybe others do also. Latest Guns & Ammo had a short piece on this - author Patrick Sweeney.

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Choke is not a single measurement but a difference .... between bore and constriction. Bores can vary slightly from ''textbook'' dia and so on any given shotgun, the bore needs measured first.

Use a reliable dial caliper and also have a "Telescope Gauge" - also known as a "Spring Gauge". "T" shaped deal with one part of top of "T" spring loaded. At base of "T" is a locking piece.

Release locking on the gauge and compress "T" end to insert into bore, back from choke area if fixed choke - keeping everything square to bore line. Lock carefully to avoid loosing the setting ........ then let fall down the bore onto a soft rag or similar. If a bore with screw-in chokes (and so true cylinder), withdraw carefully from muzzle end if practicable but care with threads.

Measure across that gauge then with caliper and record dia. Next - measure choke area or max constriction of a screw-in choke ....... either with same gauge or the ID prongs of caliper. Subtract the actual bore dia from choke constriction ....... the difference is constriction in thousandths of an inch.

Every five thousandths of an inch difference is a ''choke name'' or range, such as ''improved'', "modified", "skeet", "full" etc. Obviously the greater the constriction, the tighter the pattern is held together when it leaves the barrel.