Found this on another Ontario Outdoors Website
Question 63:
"You may not use a rifle of greater calibre than .275 ...for hunting small game in the counties of ...".
This would infer that it is legal to use a rifle commonly referred to as a .270 Win to hunt small game in the specified counties. However, the actual projectile diameter is .277".
How does the MNR define calibre (projectile diameter, bore before rifling is cut, groove to groove, land to land, etc.) ? & on what value does the MNR determine compliance (actual measurement or the data stamp on the barrel (ie. .270 vs .30-30)) ?
Asked August 22, 2000
Answer from the MNR
The MNR stance on this section is that we use the nominal calibre of the firearm in determining compliance. That is, even though as you have correctly suggested, the actual dimensions of a .270 cal. shell are .277, we accept that rifle as being a .270 and therefore treat it as a legal firearm within the restricted areas.