You are misinformed about the way they work. I understand the train of thought, but if it pierced armor due to not "sticking" to it, then it wouldn't stick enough to bite into the armor in the first place--it would slide off (ricochet), defeating the purpose.My sources are military, law-enforcement, and paralegal friends. Some of the military and L.E. ones have been involved in the testing of munitions including teflon-coated armor-piercing rounds, others in the procurement of supplies in general, including armor-piercing rounds. In some jurisdictions they are illegal, in some they are perfectly legal.
So what about armor-piercing in Sengoku-Jidai? It's kind of irrelevant really. You can fire a standard ball (lead) .22LR at 100 yards against authentic heavy-gauge steel armor and it will penetrate within 3 rounds. This is not a particularly powerful round, and neither is it heavy nor fast. The muskets of the day tended to be around .60 to .80 calibre. At the speeds and ranges encountered in the era, all but a direct hit would penetrate. Even without penetration you have the possibility of injury from shearing or blunt force trauma. And even failing that, the energy is still transferred. Any of you gents been thrown violently from a horse before? I haven't, but if I remember right there's this guy we all know who was: Christopher Reeve. We all know how that turned out.
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