Let's see..... since some seems to not notice page 6 and 7 of the opinion that specifies the size... how about another place. In the opinion he talk about a diameter of .814 which exceeds the 20mm and he lists models and diameters and concludes with ...
https://www.csaaa.org/wp-content/upl...e-shotguns.pdf
"In my opinion 12 gauge shotgun with the screw in systems for use of "Winchoke", "Browning Invector", and "Beretta Optima and Optima Plus" are prohibited under the definition in Section 9 of SOR 2020/96
These noted barrel systems are presently prohibited firearms as defined in SOR 2020/96.
" - Edward L. Burlew LL.B.
So to validate it I found a page for installing a Winchoke...
https://ktgunsmith.com/choketube.htm
... now let's look at the tool and it's cut.
https://www.brownells.com/gunsmith-t...prod25287.aspx
SPECS: 12 gauge has .814"-32 tpi.
... now convert .814 ... 20.6756 .... ooops .....