Cap'n Kobold
Hero
Actually guys I'm not REALLY looking to make guns more lethal. That might be a side effect of what I'm looking to do though.
What I want is for armor to become (or start to become) more obsolete and to advance technology past the medieval era.
To give it a more renaissance feel.
Realistic was not the right word. I don't give two #$#$ about realism. What I am looking to do is advance things along to a later type time period than medieval.
So what sort of changes do you want to evoke? Are you trying to advance one of the existing settings, or is it a homebrew?
What sort of thing is a "renaissance feel" for you?
Changes in the social laws and structure might do what you're after better than rules changes. Making it illegal (or even worse, a social faux pas) to wear armour, large shields or military weaponry in towns and cities would create the right feel; more people using civilian weapons like rapiers and light or no armours. In times of war of course, and possibly when adventuring outside civilisation, people would still strap on a breastplate and military sword if they owned one.
Increase the size of cities so there are room for several noble houses, and impose law by the ruler to prevent them from openly warring and you will get them feuding through covert actions and a duelling code.
Regarding firearms themselves:
Renaissance era musket balls are still stopped by armour. Indeed armour was "proofed" by firing a shot into it at short range. They are also often less accurate, and very much slower to load and fire than even crossbows, let alone longbows. The reason that they were adopted by almost all of the militaries was their capability for large-scale and sustainable warfare.
Both bows and crossbows require ammunition that is hand-made by a skilled craftsman. Iron or steel tips by a blacksmith, quality, straight wooden shafts, carefully aligned flights etc. These craftsman must be supplied and supported as part of the army and the components of the arrows or bolts bought, made and paid for - all for money that could otherwise go to more soldiers.
Gunpowder and musket balls can all be made cheaply in large quantities by relatively unskilled labour. This allows many more soldiers to be equipped and fielded in an army. With capability to march becoming more important, and all of a soldier's equipment being provided, armour for foot troops stops being common. With more emphasis on fighting at range, cavalry becomes a fast and mobile flanking unit rather than heavy elite shock toops, and so the requirement for armour for the upper classes reduces as well. A breastplate heavy enough to stop bullets that can be made to standard patterns to fit anyone replaces the full suit of plate and chain.