W-54 isn't here anymore. So instead US have tuned down W-76 into 5-8kt. I.e. getting new capabilities without testing (there are recent tuned down, though not that low, versions of B61 too). And that is open information. One can expect that classified would be at least a step ahead, i.e. something like 1kt. Coupled with high precision delivery and earth-penetration designs (that US has been using across the range - from conventional to B61) that makes for extremely effective deterrence as it allows to take out a dictator like Putin deep in his underground bunker if he crosses the line (Putin becoming the first and primary target himself is the cornerstone of the current deterrence architecture), and other strategic keypoints without initiating full scale war.

> Putin becoming the first and primary target himself is the cornerstone of the current deterrence architecture

Do you have any reference for this?

I'm familiar with US Nuclear bunker buster bombs. But I've never seen any writing that makes the claim the US deterrence policy is targeting Putin.

Indeed [1] makes the claim that the US in incapable of successfully attacking Russian command and control bunkers and seems from a reasonably credible writer.

[1] https://www.realcleardefense.com/articles/2020/12/01/putins_...