The person that manufactured those weapons and the bullets you are talking about is an Ironman villain named Justin Hammer.
Ironman's armor is of course bulletproof, so he specifically designs weaponary powerful enough to pierce Ironman's suits. So its not out of nowhere... its designed to be a bullet that will pierce something bulletproof.
Justin Hammer - Wikipedia, the free encyclopedia
They actually showed him in Ironman 2. He is responsable for many of the weapons attatched to War Machine's armor... so imagine War Machine shot Luke Cage with a missle. Thats basically what its like being hit by that bullet.
The same person that crafted that bullet designed the Crymson Dynamo armor that Ironman & War Machine had to team up to fight in Ironman 2... so basically its like Like Cage being shot by an Ironman level weapon not just a modified street gun.