well if the guns going to be "OMGZ DMG!" then maby it will be needed to balance it
It's a silly way to do it, though. As far as possible, balancing factors should be something that makes physical sense.
Simplest way to balance guns having a high damage per shot is to make them take a long time to reload, just like guns of that level of technology did in real life. Call it, say, a 3-4 second recharge time (still much shorter than the "six shots a minute if your troops are good
benchmark that was set in Napoleonic times, but short enough to not be tedious) with an appropriately increased amount of damage compared to bows.