Autocannons are awesome. Manny shall now perform his greatest feat yet: telling the soldiers why our firearms are low powered.
Steel bullets would merely pass through most of what humanity has to offer, inflicting minor wounds. The fracturing/melting/splattering of the lead in bullets is what does most of the damage, as the shards of lead spread out from the impact site, tearing and ripping through soft tissues and severely damaging bone. Hollow point bullets, which are now outlawed, capitalized on this damage. They inflicted devastating wounds, the type that no man's gaze settles on for long. Which I will demonstrate, with this ballistics gel, conveniently placed downrange. (At this point, Manny draws the same deagle from the last demonstration, plugging a bullet dead into the center of the dummy). Drag that thing back out here, will you? Thanks. See here, ladies? How the bullet, once it impacted the surface, riddled the inside? There is no straight "bullet hole". Now. Compare your weapons. (Coolly turns, pauses just long enough to stop his momentum, then unleashes steel hell into a second ballistics gel mold) (assistants wheel this one around too, in-case it isn't totally obvious). Thank you. Now compare. (pushes the dummies next to each other) The first dummy sustained massive injuries. Look at it. He's not getting up. Now this guy (pats the steel-slug-punctured dummy, which reacts much more solidly to the force, jiggling slightly) looks to be in much better shape. (bends down behind it, peering through the hole left behind, slightly larger than the bullets). See? Straight shot. Didn't even deflect at all. (motions to the back wall) the slug is buried in that wall, but don't worry about it. It's mostly intact, if a bit flat. This man would probably survive. That is, if I hadn't shot him right through the lung. Internal organ damage is severe, but limited. Bleeding would be significantly less than in the lead bullet case.
So you see, while our steel bullets are the only thing for the job, they are not the most effective of rounds against soft targets.