So, apart from the iron sights and rifling that were present since the 15th century, another thing I found in my search are numerous surviving revolver mechanisms like seen in this (image)thread I posted earlier;
Now, irons sights and the revolver mechanisms were basically abandoned until centuries later while rifling was usually limited to a few number of firearms.
Why was this so?
Was it solely the issue of cost?
Because a concept is not the same as being reliable and being able to mass-produce it. Lots of concepts were stumbled upon early in history - for instance Leonardo Da Vinci had the concept for an armored tank - but it was simply not possible for him to make one with the technology of his day.
With regards to repeating firearms the main issue was that metallurgy was simply not advanced enough to support firearms that had high rate of fire. Barrels had a very real chance of exploding or breaking apart even with normal ammunition loads in the 1500s, and this was exacerbated when you're firing continuously. It took 19th Century steel technology to make repeating firearms practical; along with precise machine tools to ensure that the reloading mechanism worked perfectly and didn't jam or, worse, not be sealed properly resulting in the firearm exploding.
Rifling meanwhile morosely increased reload time - it was very hard to hammer a bullet down a rifled barrel - which is why rifles were usually only issued to specialist sharpshooter units. In most battles by contrast rate of fire usually made up for the inaccuracy - if enough men are sending enough bullets down range then they will inevitable inflict enough casualties to cause the other side to give up.