Toronto is surrounded by private land. And the population density is so high that many municipalities pass bylaws prohibiting discharge of firearms, or limit it to shotguns and rimfire (e.g. for shotgun-only deer seasons), and prohibit centerfire, etc....various bylaws, the fine print all matters. So even if you find a rural landowner who is cool with shooting and the property has safe sight lines and backstops for shooting long range on steel gong targets, the municipality may not allow it, depending on the fine print of the bylaws.
You can find the Crown Land in Ontario using the Crown Land Use Atlas, link:
https://www.ontario.ca/page/crown-land-use-policy-atlas
This tool is not user-friendly, and it sometimes hangs, or crashes. So be patient with it. When you pull up the map and zoom in, you will see a policy number for various units of crown land. Some are very large, some are small. Find the policy report for that number and it will provide the acceptable uses of that unit. The list of uses does not specify "long range target shooting", nor whether centerfire rifle shooting is allowed. But it usually is specific for "hunting" being allowed or not. But it does not specify the municipal rules for whether centerfire rifles are allowed. That is where you need to check the municipality rules. Crown Land has both organized and un-organized municipalities within it. The organized ones will have a contact phone number and someone you can talk to. The unorganized ones will not.
Although not definitive, if you see that the Wildlife Management Unit that overlaps your area of interest lists the deer gun season as shotguns only, that is a clue that long range centerfire shooting may not be possible in the municipalities that overlap that WMU.
Generally in Ontario, the further north you go to get away from the dense human populations, the more uses are allowed on crown land, and you will notice that the WMU's for deer and moose seasons allow rifle seasons for several months. Contrast that to a 1-week shotgun season in southern Ontario.
Where I live in NW Ontario, the Crown Land outside of organized municipalities is usually OK for long range target shooting, as long as you use safe practices. One of the key rules is do not shoot across a road, and that includes very rough old logging roads. Basically if a quad can use it, you can not legally shoot across it, even if you are way up on a hill shooting high up into another hillside backstop. We have clearcuts here that offer LR shooting opportunities, but its extremely difficult to find any safe sight lines over 400-500m because of active logging roads bisecting the trajectory path. The logging roads are built between the hills, so inevitably the clear views from up on a hill to shoot into another hill have a road down in the valley between the hills.
So finding a legally safe area of crown land can take alot of work.
During the various hunting seasons, there are also some hunting regulation rules about having a valid hunting license for the WMU on you if you are carrying a firearm that is used for that species. Its best to have the license for the species that are open if you are carrying a centerfire rifle during the big games seasons, and be wearing the required hunter orange clothing, even if you are not hunting and just target shooting steel. For me personally, I do not do LR shooting on crown land during deer and moose seasons here, due to the number of hunters out in the bush working cutovers which is the same place I would be shooting, so I stick to my local Range and shoot paper during the big game hunting seasons here, which is essentially the entire fall and early winter. The hunters look forward to their moose and deer hunting seasons all year, and I do not want to bother them or ruin anybody's hunt by LR target practice.
I lived in southern Ontario many years ago, and I sure am glad I moved to northern Ontario and the crown land shooting and hunting opportunities it offers here.