Actually a good set of steel armor can stop a bullet. They used to shoot armor, and the armor that survived was left only with a dent. The dent was refered to as a proof mark.
Sure, but which bullet? Pistols have almost no penetrating power. I can stop a 9mm pistol round with a decent sized book. Even a .45 doesn't do much, it's just too light and too short a barrel to get a good run up to high velocity. Break out a rifle and your steel armor is in trouble. A modern .308 has 3500 J of energy, five times a 9mm pistol round and 35 times the heaviest English bow. And a musket ball is so heavy it'll punch through most armor at close range. Start playing around with .50cal and now you're talking about penetrating 20mm of armor plate. Remember, the Browning M2 .50cal machine gun we still use on military vehicles was designed to fight WWI-era tanks!
And then there's the armor. High quality armor is tricky to make. Too brittle and it shatters. Too soft and it won't stop the blow. A high quality steel was expensive and rare in medieval times. Then there's the thickness. Most breastplates were only 1-2mm thick, with helmets being a little thicker. As firearms started appearing, armor got thicker to deal with them, but only an extra mm or two. After that it's just too heavy. Almost any rifle round will punch through 2mm of even high quality steel. (Wish I had the materials to test).
A pistol is largely useless on a battlefield, so I would not want to walk out in steel armor expecting to be safe from bullets.
Period long barreled firearms fired at relatively low velocities but a very heavy ball. Much heavier then what most rifles today fire. I'd expect them to be able to see off 2mm of steel at close range easily. The introduction of long-ranged, high-penetration firearms began the arms race between firearms and armor which firearms ultimately won.
Of course, this is all moot in DF terms as firearms weren't a serious military item in the west until the mid-1500s.
PS That bit about the proof mark sounds like a load of BS. It's true the proof mark indicates the armor (or more likely, weapon) has been tested, but they were made with stamps by the armorer containing their seal as an official "inspected by #12" mark. Can you imagine how silly that would be?
Gullible Knight: "See here, my good man. I wish to purchase your finest plate."
Totally Honest Merchant: "This armor is of the highest quality! See this dent? That is where it shrugged off Excalibur itself!"
Gullible Knight: "Yes, clearly that indistinct mark must have been made by the sword of kings! I'll take three."
Totally Honest Merchant: "Indeed, my Lord is too generous. And if my lord would be so kind, I have a bridge in the New World he may be interested in acquiring the deed to?"
Yeah, right.