As late as the Boer War, irregulars were armed with weapons roughly equivalent to what the professional troops fighting them had. In fact, the introduction of breech-loading rifles allowed the Boers to use a new tactic: firing and reloading from a prone position behind minimal cover. Agreed that WW1 was the dividing line.
Do PMCs buy arms through the US armed forces, or do they go straight to the producer? So if, say, Academi (formerly XE Services, formerly Blackwater) decided to get an armed insurrection going on, what would they be armed with? Small arms? Rocket launchers? Technicals? Leopard IIs? M1A2s? Harriers?
Directly from the manufacturer. Blackwater can’t buy a Harrier, but they can buy aircraft with limited combat capability like the Super Tucano (they’ve got two, I think).