Do the areas in SW Ontario with the .275" restrictions have rifle seasons for big game (deer)? I believe these areas are shotgun, muzzle loader and bow only during the deer season.
Copied from the MNR:
The holder of a small game licence may not use a rifle of
greater calibre than .275, except a flintlock or percussion
cap muzzle-loading gun, for hunting small game in the
counties of Brant, Elgin, Essex, Huron, Lambton, Middlesex,
Northumberland, Oxford, Perth and Wellington and the
regional municipalities of Chatham/Kent, Durham, the
former regional municipalities of Haldimand-Norfolk and
Hamilton-Wentworth, Halton, Niagara, Peel, Waterloo and
York and the City of Toronto.
There is restriction on caliber for small game only as I mentioned before if you want to be certain on whether do they have open season for rifle on big game for your area you will have to refer to your WMU for more details.
Basically, you can not hunt small game with caliber great than .275 in those mentioned area but you can hunt big game in any caliber as you wish as long as there is open season and of course has to be a rifle zone not shotgun or muzzle loader only area.
If any doubt just call the local WMU to verify as to avoid any confusion usually they are willing to help.
