I'd say the last time armed resistance had direct impact on law was the abolition of slavery. Most specifically John Brown's taking of a federal armory in 1859. It didn't end well for Brown, his sons, or the freed slaves involved. It did, however, cause a polarization of public sentiment that accelerated the approach of civil war. Only after the other side was ground to paste was legislation changed nationwide.
To the commentors making a Vietnam analogy - short of storming federal installations ourselves, from whom are we to get the military-grade weaponry to put up any kind of resistance? Russia?