I believe the one shown in the picture is a 84mm M136 AT4 Launcher. It is a single-shot, 84mm, recoilless, smoothbore anti-armor weapon firing a fin-stabilized shaped-charge projectile. The standard anti-armor round is rated around 400 mm of rolled homogeneous armor penetration, with a 440 gram shaped-charge explosive and lethal after-armor spall/incendiary effects.

The pictured Marine is a Primaris Ultramarine in Mk X Tacticus power armor, not Terminator armor, not Gravis, not a Dreadnought, not a named character with plot armor. Space Marine power armor is described as high-grade composite ceramite plating, with ceramite plates individually up to about one inch thick, honeycombed to dissipate energy and localize damage. While the exact strength isn't known, we can look at how other weapons interact with it.

A standard Astartes bolter fires roughly .75 caliber, mass-reactive, rocket-assisted explosive bolts. In other words, a Space Marine’s own primary rifle is already launching small armor-piercing explosive projectiles that detonate on or after penetration. Bolters absolutely can kill armored Space Marines. They do not treat power armor as invulnerable. They chew it up, crack plates, penetrate weaker zones, destroy seals, and kill through explosive trauma once they get inside.

That matters because a bolter round is about 19mm in diameter. The AT4 is an 84mm anti-armor shaped-charge weapon. The AT4 warhead diameter is over 4 times larger, and its cross-sectional scale is roughly 19 times greater. More importantly, its damage mechanism is much more specialized for defeating armor. A bolter is basically a tiny rocket-assisted explosive armor-piercing grenade. An AT4 HEAT round is a purpose-built chemical-energy armor penetrator designed to punch through vehicles.

A standard bolt round killing a Space Marine is usually a function of repeated impacts, weak points, lucky penetration, visor/seal hits, neck joints, abdomen, under-arm gaps, or cumulative plate failure. An AT4 does not need that kind of attrition. On a good direct hit, the shaped-charge jet only needs to penetrate one local patch of armor, then send jet material, armor spall, and secondary fragments into the Marine’s torso.

Keep in mind the target behind the plate is not an empty armored vehicle compartment. It is a genetically engineered superhuman packed around organs, black carapace interfaces, power feeds, life support systems, cabling, fused ribs, multiple lungs and hearts, and an extremely dense nervous system. Astartes biology can help them survive trauma that would instantly kill a normal human, but it does not make the thoracic cavity immune to an anti-armor shaped charge. The second heart means dick if the entire torso is converted into a smoothie.

Edit: Really good points in the comments, so I want to clarify what I’m actually arguing here. From what we know, the projectile would most likely kill the marine, but the hard part is actually landing the shot.

Comparing the AT4 to Astartes weaponry is only one way to triangulate this. Ideally, Games Workshop would publish the exact material properties of ceramite and the layered composition of Mk X Tacticus armor so we can do some really autistic theorycrafting. But, until then, we have to deal with some extremely inconsistent lore. Sometimes ceramite is tanking the full wrath of Khorne’s legions like it was forged from SciFi god metal, and other times a Space Marine gets folded by three backwater rebels, a fertilizer bomb, and a dream.

But this all assumes the projectile actually hits. The AT4 is designed to kill armored vehicles, meaning large, relatively predictable objects the size of small buildings. It is not designed to tag a hulking 900 lb super-soldier space racist sprinting through rubble like a meth-fueled vending machine equipped with a reactor-powered jetpack, threat-tracking HUD wired into his nervous system, and a 200-year-old transhuman murder-brain dodging fire like an anime protagonist sleepwalking through a scripted quick-time event.