The gear generally requires an industrial base to keep it functioning. Given what was in SmarterEverDay's recent video about a barbecue cleaner, where so much of it ended up being imported despite their efforts (including chain mail!), I think in the event of another actual civil war, the USA would struggle to self-maintain any weapons more advanced than what you had in what is now "the" Civil War, and would be dependent on the whims of whichever foreign power wanted to support whichever sub-group.