I very strongly doubt we'd fight a civil war over guns unless it's a government attempt to take guns that leads to right wing protests that leads to rightists shooting guns which leads to leftists shooting guns. But the thing is, I can't see any incoming government doing something that stupid unless we got a progressive version of Trump. Like, you know, Jill Stein.
If we're fighting a civil war anytime soon, it's going to be because of an economic catastrophe. Like the 1% pushing things so dangerously far in a time of crisis (like the US losing an otherwise extremely winnable war) that it leads to an uprising that forces state governments (and possible the federal government) out. Or if states try seceding again. I could somewhat see that happening in a timeline where Trump was as smart and mentally sound as Ted Cruz and then he winds up losing the 2020 elections to someone like Bernie Sanders. Because in that situation, Trump would be too savvy to control. He wouldn't use Twitter like a 9 year old who just discovered the platform— I think Trumpists even said that his Twitter habits are most universally undesirable part of his presidency because literally everyone wants him to shut up and stop digging holes. He'd be more like the strongman PhoenixRu claimed he'd be, and he'd be able to juggle an approval rating rivaling Obama's with a gradual descent towards authoritarian conservatism rather than a rapid snap towards it. Then, in 2020 or (more realistically) 2024, he'd lose to a progressive challenger and the South would react like the Bolsheviks just took over.
If he were removed from power today, I can see plenty of his supporters mooing over it while his detractors chirp like birds for months, and there will definitely be surges of gun, vehicular, and bomb violence. But civil war? Even over guns? Ehhh.... we're just nowhere near that level yet.