You asked, "Are Winchester Magnums ever used by mass shooters?" I'm guessing no.
You said, "I think magazine capacity and muzzle velocity are good places to start a conversation here, one that could lead to the decrease in casualties from these shootings while still protecting the second amendment." I think magazine capacity is the only factor you mention that may help. But, in the end, you need extra guns at the scene. Otherwise, it won't work.
You said, "If we could have a rational conversation. Which obviously we can't." Muzzle velocity, by itself, if it is even a factor, may be responsible for .00001% responsible for deaths. It is kind of like when the gun banners went after bayonet lugs and flash suppressors and pistol grips and trademarks. People who knew how guns work were like, "Wait, what?" Total waste of time and money, even if you win and get a bill passed. Go after something that counts, like magazine capacity.
The problem is that the Second Amendment more likely protects 30 round magazines than anything else. I think you guys really need to repeal the Second Amendment if you want to do something.