Basically projectile weapons have had to move on to using larger heavier projectiles to cope with shield and armor tech advancements. It's why the vanquisher hand cannon, magnum pistol, eagle pistol, hammer pistol, myr's leg rifle, and bothrioc rifle all explicitly mention either 'large caliber' or 'magnum cartridge' i would assume that this principle extends to all projectile firearms. I can just imagine that the secure mp is using 45acp or 10mm type rounds where as everything else is either 44 or 50ae. This wouldn't be as pronounced in rifles as the extra powder and longer barrels allow for higher muzzle velocities with higher kinetic impact on target while retaining a smaller caliber, likely most modern non myr rifles would use a 30 caliber bullet with a good bit of powder behind it, and this doesn't even take into account whether or not propellants themselves haven't changed to a more efficient formula, because as it stands modern powder is nitrocellulose which is essentially nitroglycerin and sawdust (really oversimplifying this) who knows savicite could be being used as gunpowder and that shit's starship fuel