Grenades a clear requirement for a modern infantry are also banned, thus eliminating any argument that a modern standards of military efficiency apply.
Banding heavy machine guns yet another invention after the constitution was written didn’t, so there’s clear present this wouldn’t either.
Except "it was made after the constitution was written" is a standard you've made up -- there is existing case law from SCOTUS that 2A protects guns "in common use"
What makes you believe that grenades are banned in US? They are heavily taxed, yes - $200 per grenade - but they aren't banned on the federal level, and there are people who legally own such things.