Assuming a civil war, the side a given government wants to support might not have the backing of the army. And honestly, in a war scenario a bunch of dudes with AR-15s are going to do jack shit against tanks, UAVs, and actively defended military installations. Few people have TOWs at home, much less anything BVR capable. The army, meanwhile, is perfectly capable of burying a militia under missiles from a hundred miles away.

Given what we've learned from Ukraine, the most reasonable weapon to smuggle into a warring USA would be tons of cheap suicide drones. Small enough to possibly get in unnoticed (unlike, say, an MLRS battery), capable of taking out armored vehicles and strategic targets, cheap enough to manufacture at scale.