They barely existed when the Amendment was written if at all. Arms are actually the infantryman's long arm ("Battle Rifle") and firearms had been around for over 300 years and clearly were an evolving technology, up to and including the belt fed weapons of today as suitable for the citizen soldier to own, but not I think grenades or mines. Too problematic in the employment of such devices. Those probably fall under the Letter of Marque mentioned elsewhere in the document.
They’re individual weapons
Letters of Marque then, which probably referred mostly to ships, were armed with crew served weapons, which at that time and for a while after were cannon.
If they intended only for firearms, they would have written that into the Second. But they didn’t, and they preferred us to have “every terrible weapon” of war. Thus, they wrote ‘arms’.
The "individual arms only" theory is commonly bandied about by people insufficiently familiar with RKBA.
There is absolutely NO evidence that the Founding Fathers meant to exclude ANY weapon from the term "arms". Nay, the "Letters of Marque" merely provides written Congressional authorization for one to essentially wage war across international jurisdictions; that the one receiving such permission already owned such massively crew-served arms needed for such an undertaking was a given.
They barely existed when the Amendment was written if at all.
Surely you're not sitting here pontificating on this subject without having the slightest bit of knowledge about it, when Google is free of charge?
Hand grenades were invented in the 1500's, about 250 years before the Amendment was written, in fact. Explosive land mines were used in 1277 - 500 years before the Amendment - in China, even ones detonated autonomously by enemy movement. Fuse-triggered explosive cannonballs, though dangerous to the cannon's operator as well as his targets, were used in the 1400's.