Funny stuff. Almost as twisted as the NFL trying to define what a "catch" is. The more they try to define it, the farther from reality it is.
One of the definitions of an "arm" is......... A weapon, especially a firearm: A weapon. pretty simple.
Take that further, people have fought pitched battles with whatever they have at hand for a weapon. Rocks, clubs, chain, knives, pitchforks, boiling oil, machetes, guns, baseball bats, slings, arrows, bombs,....... basically anything that you can use to harm another person who is trying to harm you. That is the whole point of the 2nd Amendment, that a citizen may arm themselves to defend themselves. For that to mean anything, you need to be able to use whatever is at hand to defend yourself and it isn't up to the government to define what you use as a weapon to defend yourself. Anything you use to defend yourself is a weapon, even knowledge such as a martial art. Your hands and feet and knees and elbows are weapons and as such, arms.
Don't believe it? Use anything listed above in a fight or an attack and see what you get charged with. Assault with a dangerous weapon. Go after a cop with a parring knife and see what the charge is if you survive. pretty sure you will be charged with assault with a dangerous weapon. Which is the same as arms according to the definition in the dictionary.
I don't think the founding fathers intended for their words to be parsed. They were pretty straight forward in their thinking. If you read their comments during the process of writing the constitution you can see that.
The court is reading way too much into the writings of our founders and are creating more confusion than is necessary. Maybe on purpose. Make things too confusing, then we need them to figure out what it all means, that which doesn't need figuring out.