The reason I haven't stickied this is that there are some things with this story that don't check out for me. NO missile has ever destroyed a buried bunker at a depth of 400ft. So far, only gravity bombs have done that. From a weapons design perspective, the design approaches of a bunker buster and a hypersonic missile couldn't be more opposed to each other.

From ChatGPT:

The Kinzhal missile is a Russian air-launched ballistic missile that was first unveiled in 2017. It is a hypersonic missile designed to be carried by Russian fighter jets such as the MiG-31. The Kinzhal missile has a reported range of up to 2,000 km and can reach speeds of up to Mach 10, or 10 times the speed of sound.

While the exact capabilities of the Kinzhal missile are not publicly known, it is designed to be a highly maneuverable and fast missile, which makes it difficult to intercept by existing missile defense systems. However, it is not specifically designed to destroy buried bunkers. A bunker buster looks like this:

Inside a bunker-buster, there are several internal structures that enable it to penetrate the ground so well. These structures include a heavy and dense metal casing around the explosive charge, which provides the bomb with its weight and momentum. The metal casing is usually made of materials such as depleted uranium or tungsten, which are very dense and heavy.

In addition to the metal casing, the bunker buster may also have a hardened nose cone made of a similar material, which is designed to withstand the initial impact with the ground or concrete. This nose cone helps to keep the bomb on target and prevent it from veering off course.

The explosive charge inside the bunker buster is typically surrounded by a layer of high-strength steel or other material that is designed to direct the force of the explosion downward, further aiding penetration. Some bunker busters may also have a delayed fuse or timer, which allows the bomb to penetrate deep into the ground before detonating, increasing its destructive power.

Guaranteed, the Khinzal would have none of this.

It is worth noting that destroying a buried bunker at a depth of 400ft would be a difficult task for any missile system that emphasizes speed and maneuverability (the Khinzal would be basically a small SRB with a 1000lb max warhead), as it would require a high level of precision and a large amount of explosive power. It is also possible that the bunker may have been designed to withstand such an attack.

There is a chance this story could be true if the Russians used one of their new glide bombs on the bunker. They are GPS guided, have warheads of 1500lbs, and can coast 65km to their targets (launched from 14km high). But those bombs are not bunker busters. So, hence, I'm having a hard time explaining this feat by Russia. I still think that "hearing buzz" about this from multiple channels means it's likely true, tho, but, I hesitate to sticky this based on these doubts.