If you consider what humanity has to fight in the 40th millenium...
Overly-exaggerated green-skinnede football hooligans that want to kill you and have the ability to survive decapitations because they are partially fungi, and can pierce any armor they feel like just because they don\'t think your armor is good enough protection against their bullets and axes
Hordes of giant bugs that have the ability to evolve in the timespan of minutes to become immune to your weapons, who want nothing but to eat you and everything around you and your planet until it is nothing but a barren asteroid in space, and come in numbers so massive they have the combined mass of a galaxy
Robots made out of self-regenerating metal harder than any material known to man even in the 40th millenium, that have inertialess-drives on their space ships, the ability to phase out of existence to avoid taking damage, the ability to teleport when they take too much damage, the ability to get back up even when they are reduced to a tiny puddle, and guns that shoot through multiple dimensions, piercing any kind of armor and diassembling your molecules into their base atoms
Your own brothers, who happen to have better equipment than you because their equipment is tainted by chaos
Gigantic warp daemons that are made out of materials like fire, which furthermore makes them immune to mundane weapons like bullets and require you to use special weapons against them
Space communists with extremely powerful ranged weaponry, giant robots and rail guns
Space elves that are so fast that they can casually dodge bullets, even giving super soldiers a hard time keeping up with them, shooting monofilament or razor sharp poison crystals that kill you instantly, that also happen to be extremely powerful psykers that are mostly immune to the perils of the warp because they have fancy helmets that you don\'t have
Then .75 caliber self-propelled mass-reactive diamantine tipped depleted uranium cored bolts make sense