as for 30-30, here is where i can dork out a bit. <:KEK:726877368601411624> the 30-30 designation stands for 30 cal (American 30cal *usually* = .308 ) loaded with 30 grains of smokeless. ofc, that was the original smokeless powder Winchester was using, and it varies from load to load based on powder type, bullet size, and case length, amongst other things. Guys that have single shots or stacked-mag guns often load .308 spitzers in 30-30 cases for better accuracy too, where as most 30-30 specific bullets are the same diameter as .308win bullet, just with a blunt nose for use in a tubular mag. ofc, grain weight also comes into play, and is probably the thing that limits compatibility of bullets between 30-30, .308win, and other similar cartridges the most. e.g., you would not want to put something like a 250 grain AP bullet into a 30-30, as 30-30s were never intended to throw that much weight, nor would you be able to get the velocity required to actually do anything with it, but that is probably common sense. xD
