Earlier quoted context omitted.
There's an old saying in Tennessee — I know it's in Texas, probably in Tennessee — that says, "The only way to stop a bad guy from swatting you is a good guy with his own personal SWAT team."
Own a musket for home defense, since that's what the founding fathers intended. Four ruffians break into my house. "What the devil?" As I grab my powdered wig and Kentucky rifle. Blow a golf ball sized hole through the first man, he's dead on the spot. Draw my pistol on the second man, miss him entirely because it's smoothbore and nails the neighbors dog. I have to resort to the cannon mounted at the top of the stair…
Specifically, even in 1787 it was seeming likely that there was a good chance that at some point the federal government was going to abolish slavery by force, and the framers of the Constitution needed the southern states on board with the Constitution's stronger central government vs. the Articles of Confederation.
So, they probably intended weapons of some proportionate power compared to contemporary military weapons, but the ultimate underlying motivations are suspect.
On the other hand, I could imagine a world where in 1787 alcohol and/or marijuana were legal in some states and illegal in others and the framers were worried about federal troops imposing national drug policy on drugs that never crossed state lines. In that case, it seems more reasonable, which I guess lends respectability to their reasoning at a medium level of abstraction.