Simplify the following text for a Grade 7 native English-speaking person.
Original: The American gun market produced several famous dangerous game cartridges around this time , such as the .458 Winchester Magnum , .378 Weatherby Magnum and .460 Weatherby Magnum and many of these were ` wildcatted ' ( to modify an existing case and rifle to fire a different caliber bullet ) .
Simplified: The American gun market made several famous dangerous gun cartridges around this time , such as the .458 Winchester Magnum , .378 Weatherby Magnum and .460 Weatherby Magnum . Many of these were ` wildcatted ' ( to modify an existing case and rifle to fire a different caliber bullet ) .