30 caliber does not require a different lower. Round length is what dictates which lower you must use. If you will sign up and read that thread you can see which calibers can be used on which size lowers.
7.62x39 & 300 aac blackout are both 30 caliber rounds that work with an AR15 lower receiver. 308 is a 30 caliber round that requires and AR10 lower receiver.
7.62x39 & 300 aac blackout are both 30 caliber rounds that work with an AR15 lower receiver. 308 is a 30 caliber round that requires and AR10 lower receiver.