Learn something new everyday. Thanks for the schooling.Not completely true.
.223/5.56 mags will work with a few diff't calibers - to an extent. Some will just reduce the total capacity of the magazine due to the larger round, others should be used with a dedicated magazine in that caliber.
Example, .223/5.56 magazines will work with 300BLK, no loss of capacity. No issue with feeding, for the most part - can vary by magazine type.
6.8SPC should use dedicated 6.8 mags.
7.62x39 uses a dedicated mag.