I agree with you, but where do you draw the line? In Iraq and Afghanistan there are children who own and know how to operate 20 year old Russian RPGs. I invite anyone to go to Afghanistan, where every male over age 10 has a Kalashnikov. And over there, an armed society ain't exactly a polite society.
"...being necessary to the security of a free State..."
One might also consider that the Afghanis and their Kalashnikovs held off the Russian Army for many years.
Original intent.
The founding fathers did not have RPGs, so they could not have intended for the Second Amendment to apply to RPGs.
They did, however, have fifty-caliber rifles.
Under the second amendment, you draw the line at individual misuse, not at mere possession. Arms is arms, and that includes RPGs and MANPAD SAMS. Both bearable by a single person, although that is not a requirement, since the 2nd protects keeping as well as bearing. When written it protected possession of privately owned cannon, both field pieces and shipboard.