Well most countries, only a small fraction of the stockpile is even ICBM capable. So if you have 300 warheads, only 50 might be able to reach another continent. Of these 50, maybe 25 are destroyed on the ground or intercepted. That only leaves 25, and if those are going towards military targets, then no, it's not really enough to devastate another country.It's always weird to me when people talk about a country having *only* a few hundred nukes. As if that isn't already enough to devastate another country.
Even if the top 25 cities in the US were each hit by one nuke, the vast majority of the population would survive. It's certainly not the same as Mutually Assured Destruction (MAD).