Assuming that we need maximum lethality on an armoured target, with collateral damage being considered highly desirable, the most logical ammunition is a long-rod penetrator. So, we base this ammunition around a 5x~45mm uranium needle (so as to fit inside the 51mm length of the ammunition without fouling on the cartridge's base & primer) with a non-discarding finned jacket filled with 4g of Polonium-210 powder, functionally an APCR/HVAP projectile. We fill the 7x51mm NATO brass with a HNIW-based propellant in order to achieve the maximum muzzle velocity.
We fire this ammunition from a weapon that looks like the required Mk 14 EBR, though we modify the rifle to have a much stronger barrel and to be smoothbore, since long fin-stabilised ammunition is faster and more damaging when not spin-stabilised. We load the brass to the point where mechanical failure of the weapon is possible after a single magazine of these rounds are fired, in order to achieve the maximum muzzle velocity, assuming that the shooter is sufficiently protected against mechanical failure of the weapon, or is a suicide agent.
So, we have several avenues of lethality. We have a cartridge loaded to produce very high muzzle velocities, at the cost of effectively destroying the rifle within a single magazine-worth of shots. We have a jacket filled with Polonium dust, that will be stripped off the depleted-uranium penetrator on impact and will burst, filling the air around the victim with several grams of Polonium-210 dust, which has an LD50 of .089 micrograms The depleted uranium penetrator carries most of the kinetic energy on into the victim, passing through any practical thickness of titanium armour (if any) and most likely disintegrating since uranium is brittle, delivering the remainder of the kinetic energy to the target's vulnerable organic or electronic vitals.
So, we have a cartridge that can literally kill millions, mainly from the Polonium-210, assuming that the Polonium disperses widely enough, and considering that the Polonium is concentrated in a relatively small area, we can assume that - provided the Cyborg leader's fellow regime members breathe - they will die within days, having received a lethal dose of radiation within minutes of inhaling the stuff.
A standard 7.62 NATO M61 AP round can penetrate about 13mm of steel armour at its muzzle velocity of about 840 m/s, and at 2000 m/s, the armour penetration capability of a ~45mm depleted uranium flechette would be much higher, and using Newton's Impact Depth approximation, Depth ~= Length x (Projectile Density/Armour Density), potentially penetrating up to 112mm of steel armour, or 190mm of Titanium.
It is most likely not practical to armour a human-seeming or even a humanoid cyborg with Titanium to a degree that this ammunition would not be able to penetrate, and even if the projectile does not kill the target outright, the Polonium-210 most likely will.