Who has the gun out and where are we? Because there are a lot of places you should be rushing the person bringing out the weapon before it can be brought to bear, to say nothing of if the agitator is actually brandishing the weapon. Because of how fast these can take place, initiative should be called for if someone was watching the caster with suspicion when he tries to cast.If you were to see a man holding a firearm, would you immediately charge and attack him?
They don't know you and unless you deceived them ahead of time, they should be acting with suspicion if you start chanting gods know what in a powerful voice and making weird hand gestures. Doing that is like showing off what could be a remote control detonator to someone who doesn't know you.Likewise, magic in D&D has a myriad of beneficial effects as well, so the average PC/NPC would probably not attack a caster until they took a hostile action against them - they couldn't be sure until the end of the casting, anyway.
two points difference of DC does not make the sell useless.Is it supposed to be near-useless in the default dungeon situation
You pretty much have to beat your allies in initiative to get the best use out of charm person as well.or is it a spell that could work well if you get it off before they act and then talk to the rest of the party instead of fight?
If you want to be able to control people subtly, learn to cast subtly. As in using the Still Spell feat, and or Silent Spell feat, or just knowing the spell's range, and how many minuses your targets get to their Spot and Listen checks at that range.