In our world, plate armor was anything but "common". It survives well and so it appears that everyone wore this, but the modern vision of fully-articulated armor of nearly all-metal construction was very rare and only affordable to the ultra-rich who could spend that much on something dedicated only for either parades or warfare, and custom forged for themselves exclusively. It took months to construct these suits so that they would allow enough mobility to be useful, and that utility varied widely. Again, this was quite expensive.
You didn't want to fall into a pond or a deep patch of mud wearing this stuff, either. See https://en.wikipedia.org/wiki/Frederick_I,_Holy_Roman_Emperor, and the many French knights who fell from their horses at Agincourt and drowned face-down in the mud. Some suits of heavy armor were specifically designed to be worn only on horseback and were so heavy that the knights needed cranes to hoist them up onto their horses. These, along with the horses, were the battle tanks of the time.
In reality, most metallic armor was constructed of bands or scales sewn onto padded cloth or leather, or chain mail suits with a few strategically placed plates that offered little protection against blunt force. The punch of a .45 round provides plenty of blunt force even without penetrating this kind of armor (which it likely would). Even mail armor was rather expensive. Aside from that, the rest of anything that can be considered armor would have been cured or soft leather and padded cloth.
It is possible to forge hardened plates that would stop, or better yet, deflect this projectile, but the resulting armor would be terribly heavy. What is the metallurgical level of your world? Do they have the skill to harden steel, and how good is the steel they make. 347
Material tech is important here, but plain old mild steel would need to be thick, and therefore heavy. If the steel and hardening techniques are available, one might be able to wear this on their chest and maybe as a helmet, but a full suit of it would be very difficult and clumsy to move around in on a battlefield and would afford a marksman plenty of time to aim for a soft spot, and it would be exhausting to wear for any practical amount of time. Many medieval battles lasted for hours, if not days. The armored knight would be a sitting duck, essentially.
I don't want to write your story for you, but maybe your denizens would figure out that pistols need time to be reloaded, and maybe they could figure out that layers of wood and sheets of steel would make fantastic tower shields to stop initial volleys of bullets that once survived make the shooters the sitting ducks to be butchered a bit later. The Greek phalanx made effective use of this sort of wall o' shields; that's an old trick. Tactics win, armor and weapons are not the only keys to victory. The point is that softer materials and layering might be discovered to be more effective at absorbing the kinetic force of a bullet than single plates of metal. Shields can be dropped, and the warrior behind it then has good mobility to advance and slay a reloading pistolier (but they'd better hurry!).
The English longbow and bodkin points on the arrows essentially invalidated medieval armor and ended the "arms race" between weapons and those types of armors. Once full suits of armor became obsolete, the focus shifted to mobility. There were plenty of breast plates worn right into the 19th century (some armor is still better than none), and generally by cavalry forces, but due to cost considerations, the growing size of armies, and the change in strategy to speed over brute force, combined with the improving quality and effectiveness of firearms, armor virtually vanished until material science caught up in the 20th century and it was possible to construct plastic armors that would stop bullets. When you get hit by a bullet wearing this stuff, it still knocks you on your butt, can break ribs, and so forth. Bullets pack a whallop. 347