The first explosives to start your people off with are hand grenades, not big huge truck bombs or strange concoctions your science nerd whips up in his basement. Don't fool around with anything homemade. Do not buy them. Do not stockpile them. Unless you are properly trained in their use, machine guns are more dangerous to you than they are to the enemy.
Automatic weapons have two specific uses in military tactics.
One is for the defense of established positions. You do not have that kind of skill and training level. You can accomplish anything you need to accomplish to attain the initial objectives of an insurrectionary movement with other weapons. Do not flourish them, display them, or let anyone know you have them. Do NOT buy guns in excessive quantity, and do not saw off your shotguns below the legal limit until the legal line has been breached and you're going to jail anyway.