I'm assuming a purely conventional conflict since there's no telling how bad the shitshow will get if nukes start being exchanged.
So if both Russia and US were to fire their ICBMs in quick succession (mutually assured destruction) scenario, it’s likely the US ones will be a lot more potent. Most Russians are clustered on the west near Moscow.
US cities would still be hit and it would be very devastating, but Russia and China would be obliterated.
Death toll would be in the billions.