As has been said before... which Superman? The character has been in continuous publication since June, 1938, and has been adapted to every form of media short of semaphore signal. So there is the obvious problem that Superman's broader set of powers has changed with the decades, but even in a particular era, such as say 2000-2010, he'll display different levels of power depending on the writer of any given adventure, and (more importantly) the needs of the plot.
Superman, as he appears in DCA has a Toughness 18, with each and ever point of that being Impervious,plus Ultimate Effort (Toughness) and Extra Ordinary Effort. So, even if we ignore the effects of those two feats, you'll need some heavy duty ranks of the Penetrating extra or a minimum of Damage 10 to get Superman to notice.
Going through the sample weapons in the Equipment chapter, unless you have Strength 4 and a Chainsaw, you can't hurt him with any listed melee weapon; the only ranged weapon that he'll notice is the rocket launcher. Going through the sample vehicles, the cannon on a main battle tank, anti-shipping gun of a naval destroyer, and lasers of alien space fighters are only at the Damage 10 minimum. Air-to-air missiles from fighter jets, bombs from military bombers, and gun batteries on battleships (Damage 11 to 13) hit him roughly as hard as yoy or I would feel getting hit with the punch of a strong goon. Basically, of the mundane military equipment listed in DCA, Superman only has to really worry about the ballistic missiles launched by attack submarines (Damage 15). Seems pretty invulnerable to me!
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