"Federal troops can't be used in a domestic law enforcement role without a lot of hurdles being jumped."
If there is a general "armed insurrection," by an organized group, and depending on its size and its scope, it may well rise above the level of a law enforcement matter and into the scope of military response required.
At that point, posse commitatus is a moot point.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.