at .565" Diameter It is a Turkey constriction for a Remington 20 ga turkey is the tightest (most constriction) of all the choke tube over .040 it is for use with lead shot only good article on chokes here rem870.com/2012/05/06/shotgun-chokes-explained-cylinder-improved-cylinder-modified-full/

using Remingtons 20 ga bore size of .615 and the choke size of .565 it wiuld give you a constriction of .050 or 50 thousands constriction

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The closest to 6.60 is 6.61 which is for a 16 gauge which would be an Improved Cylinder choke - Here are the different sizings. - randywakeman.com/Choke%20and%20Barrel%20Specs.htm

This choke's 1.050" parallel section produces optimum patterning with your Remington Choke shotgun for turkey hunting. It is an "Extra Full Choke" [Turkey Choke].