Nobody even knows how to use a "vampire tap" anymore!
The cable company is waiving the "Fair Use" defense because they don't want to win a case that then strengthens the "Betamax" decision.
I don't think it's that difficult to draw the line. Contemporary writings to the constitution describe the armament that all militiaman should have as including a musket and sufficient quantities of shot and powder. There's no mention of the "WMDs" of the day, ie: cannon and mortars.
Therefore, you might interpret the 2nd as guarantying the rights to the weaponry of an individual infantry soldier. This would exclude squad operated weapons, but include light automatic rifles, grenades, and the smaller varieties of RPGs.
This is still not very likely to happen, but this is where a straightforward reading of the 2nd would get you.
The discussion is here and the usual gun-grabbing Leftists are already polluting it. Can you say Eisenstein
/* Halley */ (Halley's comment.)