Grenades and mines are classified under ‘arms’.
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.