How does state/federal law work with MMJ card?

I understand the basics of the 4473 and things being legal at state vs federal level but how does this actually work? Say you used your firearm in self defense and had a MMJ card, maybe state wise you're ok but what's stopping/ what's the line that would get crossed for fed to get involved? It's confusing to me that 2 laws could be legal but completely opposite. Some states say you can have a license or conceal/open carry license but that doesn't change federal law.

submitted by /u/chicitypk
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