My limited understanding is that plate-armor fell out of favor with the introduction of small arms since a bullet could easily pierce a breast-plate. Thus the extra weight became a liability and armored suits quickly became little more than decorations and heirlooms rather than practical battlefield defense.
My question then is did anyone attempt to reinforce plate suits to try and make them bulletproof? Are there any examples or records of experimentation in this regard. One off historical curiosities / attempts to make a "bullet proof knight"?
Breastplates designed to stop powerful crossbows were effective against early handguns. Until the 16th century, European handguns usually had muzzle energies of under 1000J, and the 2-3mm of iron/steel used to stop powerful crossbows would stop bullets of such energies. At this point, there wasn't any need to improve the armour.
The much thinner armour on the arms and legs (under 1mm, vs 2-3mm for the body) wouldn't stop bullets (but they also wouldn't reliably stop crossbow bolts or arrows - depending on the angle, they might deflect them, but a square-on hit could penetrate the armour).
Muzzle energies of long handguns continued to climb, sometimes exceeding 3000J by the end of the 16th century. To put this into context, the energies required to put a bullet (round ball, as used then) through mild steel plate of different thickness are:
| Thickness (mm) | Energy (J) |
|---|---|
| 1 | 450 |
| 2 | 750 |
| 3 | 1700 |
| 4 | 3400 |
So, the thicker good-quality old-style armour (at least the breastplate) would be effective protection against guns to about the middle of the 16th century, and effective against muzzle-loading pistols until their disappearance from the battlefield. The 17th century saw the end of common use of full armour, with breastplates getting thicker, in turn leading to the abandonment of iron/steel armour other than breastplate and helmet (a buff coat might provide protection for the arms and lower torso and thighs). Breastplates for cavalry reached thicknesses of about 6mm, and armour of 9-10mm thickness was used by assault troops in sieges - armour of such thickness could withstand musket shots at point blank range.
A 17th century breastplate with a bullet dent - possibly from battle, possible a proofing mark (i.e., a dent from testing whether it can stop bullets by shooting at it): https://i.pinimg.com/564x/26/aa/f7/26aaf7b5085e23c4932f574d21980bef.jpg
Full armour survived into the mid-17th century, although most had abandoned it by then. Two regiments of cuirassiers fought in the English Civil War. One of these, "Haselrig's lobsters", or the "London lobsters", was so nicknamed due to their armour already being such an unusual sight on the battlefield. At the Battle of Roundway Down, 1643, Haselrig himself prove the worth of such armour by surviving multiple pistol shots (at least 4), and being quite sword-proof. His horse fared less well, and Haselrig was lucky to be rescued. The king (Charles I) commented on Haselrig's armour:
Had he been victualed as well as fortified, he might have endured a siege of seven years.
The type of armour they wore: https://commons.wikimedia.org/wiki/File:Armure_savoyarde_IMG_3810-original.jpg - pistol proof, and useful against musket balls at long range.
Not to discourage more answers, but you should check out u/WARitter’s answer about the process of testing armor against projectiles here (it’s actually where we ultimately got the term “bulletproof” from!).
