Metralla literally translates to "shrapnel", the metal fragments from an exploding bomb. It's a very intense and unusual word to find in a romantic song!
Juanes uses it in the powerful metaphor, "Me siento indestructible como un cañón de metralla" (I feel indestructible like a shrapnel cannon). He uses this extreme, military-grade imagery to express just how incredibly strong and invincible his partner's love makes him feel. This unexpected word choice makes the feeling he's describing truly unforgettable.
Colombian rocker Juanes delivers a massive burst of romantic optimism in "No Siento Penas", and he does not hold back on the wild comparisons. The spark is tiny: someone whispering in a low voice. The Spanish phrase "a media voz" literally means "at half voice," which describes that quiet, intimate murmur when someone is right next to your ear. That whisper triggers absurd levels of power. One second he feels warm like the morning sun ("como el tibio sol de la mañana"); the next, he feels like the scariest hurricane around ("el huracán más temible") or an armored artillery cannon ("un cañón de metralla"). It is totally over the top, but anyone who has fallen hard knows that exact brand of silly invincibility.
The real joy hits in the chorus, where love cures every minor annoyance life throws at you. Juanes rattles off a checklist of wiped-out troubles: "No siento penas ni dolores de cabeza" (I don't feel sorrows or headaches), followed by zero mental fog ("Ni confusión de ninguna naturaleza" [Nor confusion of any kind]). The word pena covers everything from shame to deep grief, but here it simply means life's daily drag vanishes. Everything clears away until there is only pure, uncomplicated affection left for the person who changed his world from day one.