Firearms massively shifted the balance of power from defense to offense, which changed... Everything...
Before firearms, attacking castles and such walled fortifications required, significantly greater force than defending them did, but cannons changed that and the resulting need for larger defensive forces, forced the transition from feudal to centralized power.
Hand held firearms obviously also meant that the High Lords could no longer ride forth to slaughter the puny peasant soldier, confident that their expensive armor, weapons and training would protect them. It no longer mattered if you had the best body armor money could buy and had better training, as no amount of training would allow you to dodge a bullet, and no armor you could wear and actually fight with could offer any significant protection if you got hit... a single shot from some musket held by a puny peasant could still easily kill you...
And then we got rifled guns and telescopic sights and such... And all the security in the world can no longer stop a lone gun man, if they are willing to accept the "possibility" of being killed or captured, and if they are willing to bide their time, waiting for the perfect opportunity... We are at a point where almost anyone can kill almost anyone else, including kings, presidents, etc...
And as an inevitable consequence, we got democracy... The illusion of power for the masses, to keep us in line, by making it uncertain who our real oppressors are behind the scenes...