To the best of my knowledge the .338 can will fit on smaller calibers but the .30 cal can won't fit on the .338
That is correct; the interface is designed so that you can't accidentally put a .308 can on a .338 but vice versa works. And it's been my experience that if you shoot a larger bore diameter can than the supported cartridge, it works fine but doesn't suppress as efficiently (ie .30cal to .223), assuming you're sticking to a bolt gun and not a semi-auto.