Then who would define that arm?
Here's a hint: It doesn't need defining....
"Then who would define that arm?"
An arm in this context is a weapon. A weapon is an object utilized to defend oneself on the streets of America, in our homes, and as soldiers in war. What definition are you seeking beyond a limit to arms? Should we prevent citizens from owing nukes? Is that where you are going? Ok, can you afford one? Can any of your friends afford one? And if they can, are they prepared to become radioactive and die themselves when they mis-handle it?
As a citizen of these United States, the 2nd Amendment allows me to decide and define the arm I choose to protect my family. Not the states, nor subsequent laws passed by the fed which restrict the choice of weapon.