What's a "passed case"?
Written opinions by appellate level courts (e.g., the U.S. Supreme Court, state supreme courts, and federal and state intermediate appellate courts) are readily available through any number of sources online (as are opinions by federal trial level courts). Of course, there are literally hundreds of thousands of published cases, so unfocused searching and reading will be a bit daunting.
FWIW, the two decisions cited in the prior response are of marginal importance -- even to most of the minority of Americans who own guns. If you want "most significant court decisions of the 21st Century," then google that. You'll get a bunch of lists that will have some cases in common and some variance. It'll be far more manageable than unfocused reading.