Winchester does make the .300 Win Mag and .300 Short Mag. They're both 'proprietary', meaning that Winchester gets paid a royalty if another company uses their cartridge and name
Winchester also manufactures many other .30 caliber CARTRIDGES from other companies like Remington. Winchester also manufactures bullets, brass, powder, primers & wads
I suggest picking up a copy of
Cartridges of the World , and reading through that for some good reading and excellent information