The SAAMI spec on 20 ga bore diameter is .615-.635.
Ralph Walker listed the following constrictions in The Gun Digest Book of Shotgun Gunsmithing, 1983:
20 GAUGE Sk .004, IC .007, Sk2 .009, Mod .014, IMod .019, F .025
My Beretta 686 measures .625 bottom barrel, .626 top barrel. Briley appears to make their choke tubes on the assumption that Beretta bores are about .625, because their Skeet is .620 which would be .005 constriction, and their Mod is .610, which would be .015 constriction.
Other gun brands will vary in bore diameter. Some might even be outside the SAAMI specs, as that is only a recommendation and only applies in the USA.
There is a lot of variation in bore diameters even among guns of the same make and model. I don't know how true it is, but I have read that manufacturers use a barrel reamer made to a specific diameter, and when it gets dull it is resharpened, which reduces its diameter. When the reamer gets below a certain minimum diameter, it is replaced.
A choke tube manufacturer has to guess what the bore diameter will be on guns in which his tubes will be used. That is especially difficult when several different manufacturers use the same choke tube barrel thread design. For example, Winchester, Browning, Mossberg, Weatherby, Thompson Center, Stoeger, Smith & Wesson, SKB, Ruger, NEF, Miroku, Ithaca, and Charles Daly use the same chokes on at least some of their models, but their bore diameters may vary a lot.
Trulock told me they assume a bore diamter of .725 for 12 ga Beretta MobilChoke tubes, based on actual measurement. Briley said they use .723 - that is what Beretta claims it is, but I don't know whether Briley took Beretta's word for it or measured some guns.
The bottom line is, you can't assume any bore diameter for a specific gun - it has to be measured. Once you have that, you can ask the choke tube manufacturers what average bore diameter they assume for their tubes, or what the actual diameter of their tubes is.