The feds dont get involved in cases where there is no need or direct threat as such the average Joe will follow the laws and go about their usual business, but allowing medical mary-jane opens the door immediately to a huge market with hundreds of millions of bad perscriptions, also opens the door to massive interstate trade and commerce to nearby states, regardless of how illegal it is there.
As far as a state ignoring the gun laws feds have, in most cases it will do no harm, but it opens the door to consider any guns not purchased legally or posessed legally as a crime under federal law. Its like the saying "its all fun and games until somebody gets an eye poked out". It will work fine and dandy until someone screws up, then the feds come in to clean up the mess.
Simply put, the laws state that a former convicted felon cannot posess or own a firearm, they lost that right, Id prefer to keep it that way, and keep the guns in posession those who need protection from the criminals, after all more than 77% of convicted felons are convicted again, and again.