Maybe the vast majority of shootings involve handguns because assault weapons are much harder to get thanks to government legislation and crackdowns?
Toronto police have had numerous press conferences where they show off plenty of automatic weapons from submachineguns to assault rifles. The reason why criminals choose not to use larger more powerful weapons isn't because of strict gun laws, its because of concealment and convenience.
A handgun is something you can have on you at all times and be concealed on your body. How are you going to carry a large weapon around and be able to do the same? A gun or any weapon isn't very useful if you can't have it ready to go at a moment's notice when you need it.
If you want a simple gun law that works that doesn't penalize legal gun owners, then just make a law where if you're found to have a firearm on you without any legal license then you get an automatic 2 year prison sentence with no questions asked. Legal gun owners won't be affected by this rule while criminals will think twice about carrying guns or if they still choose to use a gun they'll be more likely to toss it rather than keep it and be caught with it.
If you really want to make it hurt, then make any crime committed with a firearm be an automatic 10 years jail time on top of whatever sentence you received for the other crimes they committed. I'd seriously be interested in seeing how many gun crimes would be committed then if such laws were implemented.